Implement first version of PagedSource

First step towards migrating to PagedSource interface. This a new
api replacing DataSource in paging 3.0. It combines the functionality of
different subclasses of DataSource under one class and differentiates
available optmizations by its KeyProvider.

As the first step, this change simply switches Pager to use the
PagedSource api over DataSource and a PagedSourceWrapper was introduced
to migrate the existing Listenable*DataSources to the PagedSource api.

Test: ./gradlew paging:paging-common:check
Test: ./gradlew paging:paging-runtime:cC
Change-Id: I1d04abdb857f686d4bde3b12dedae5cfd434dac5

diff --git a/paging/common/api/current.txt b/paging/common/api/current.txt
index cd07f54..60b49fe 100644
--- a/paging/common/api/current.txt
+++ b/paging/common/api/current.txt

@@ -293,6 +293,78 @@
     method public static <Key, Value> androidx.paging.PagedList<Value> PagedList(androidx.paging.DataSource<Key,Value> dataSource, androidx.paging.PagedList.Config config, java.util.concurrent.Executor notifyExecutor, java.util.concurrent.Executor fetchExecutor, androidx.paging.PagedList.BoundaryCallback<Value>? boundaryCallback = null, Key? initialKey = null);
   }
 
+  public abstract class PagedSource<Key, Value> {
+    ctor public PagedSource();
+    method public abstract boolean getInvalid();
+    method public abstract androidx.paging.PagedSource.KeyProvider<Key,Value> getKeyProvider();
+    method public abstract void invalidate();
+    method public abstract boolean isRetryableError(Throwable error);
+    method public abstract com.google.common.util.concurrent.ListenableFuture<androidx.paging.PagedSource.LoadResult<Key,Value>> load(androidx.paging.PagedSource.LoadParams<Key> params);
+    property public abstract boolean invalid;
+    property public abstract androidx.paging.PagedSource.KeyProvider<Key,Value> keyProvider;
+    field public static final int COUNT_UNDEFINED = -1; // 0xffffffff
+    field public static final androidx.paging.PagedSource.Companion! Companion;
+  }
+
+  public static final class PagedSource.Companion {
+  }
+
+  public abstract static sealed class PagedSource.KeyProvider<Key, Value> {
+  }
+
+  public abstract static class PagedSource.KeyProvider.ItemKey<Key, Value> extends androidx.paging.PagedSource.KeyProvider<Key,Value> {
+    ctor public PagedSource.KeyProvider.ItemKey();
+    method public abstract Key getKey(Value item);
+  }
+
+  public static final class PagedSource.KeyProvider.PageKey<Key, Value> extends androidx.paging.PagedSource.KeyProvider<Key,Value> {
+    ctor public PagedSource.KeyProvider.PageKey();
+  }
+
+  public static final class PagedSource.KeyProvider.Positional<Value> extends androidx.paging.PagedSource.KeyProvider<java.lang.Integer,Value> {
+    ctor public PagedSource.KeyProvider.Positional();
+  }
+
+  public static final class PagedSource.LoadParams<Key> {
+    ctor public PagedSource.LoadParams(androidx.paging.PagedSource.LoadType loadType, Key? key, int loadSize, boolean placeholdersEnabled, int pageSize);
+    method public androidx.paging.PagedSource.LoadType component1();
+    method public Key? component2();
+    method public int component3();
+    method public boolean component4();
+    method public int component5();
+    method public androidx.paging.PagedSource.LoadParams<Key> copy(androidx.paging.PagedSource.LoadType loadType, Key? key, int loadSize, boolean placeholdersEnabled, int pageSize);
+    method public Key? getKey();
+    method public int getLoadSize();
+    method public androidx.paging.PagedSource.LoadType getLoadType();
+    method public int getPageSize();
+    method public boolean getPlaceholdersEnabled();
+  }
+
+  public static final class PagedSource.LoadResult<Key, Value> {
+    ctor public PagedSource.LoadResult(int itemsBefore, int itemsAfter, Key? nextKey, Key? prevKey, java.util.List<? extends Value> data, int offset, boolean counted);
+    method public int component1();
+    method public int component2();
+    method public Key? component3();
+    method public Key? component4();
+    method public java.util.List<Value> component5();
+    method public int component6();
+    method public boolean component7();
+    method public androidx.paging.PagedSource.LoadResult<Key,Value> copy(int itemsBefore, int itemsAfter, Key? nextKey, Key? prevKey, java.util.List<? extends Value> data, int offset, boolean counted);
+    method public boolean getCounted();
+    method public java.util.List<Value> getData();
+    method public int getItemsAfter();
+    method public int getItemsBefore();
+    method public Key? getNextKey();
+    method public int getOffset();
+    method public Key? getPrevKey();
+  }
+
+  public enum PagedSource.LoadType {
+    enum_constant public static final androidx.paging.PagedSource.LoadType END;
+    enum_constant public static final androidx.paging.PagedSource.LoadType INITIAL;
+    enum_constant public static final androidx.paging.PagedSource.LoadType START;
+  }
+
   public abstract class PositionalDataSource<T> extends androidx.paging.ListenablePositionalDataSource<T> {
     ctor public PositionalDataSource();
     method public static final int computeInitialLoadPosition(androidx.paging.PositionalDataSource.LoadInitialParams params, int totalCount);