Replace LoadParams with a sealed class

This CL swaps the data class LoadParams with a sealed LoadParams class.
This way, we can ensure that Append and Prepend come w/ non-null keys
while still keeping 1 method interface for cases where they only have
1 direction paging hence do not care about the load request type.

Bug: 154953944
Test: existing tests
Change-Id: I61ae333e49d514dcccc632c46e1e18759ede039e

diff --git a/paging/common/api/current.txt b/paging/common/api/current.txt
index 3bcf1c4..26686b0 100644
--- a/paging/common/api/current.txt
+++ b/paging/common/api/current.txt

@@ -274,19 +274,27 @@
     property public boolean jumpingSupported;
   }
 
-  public static final class PagingSource.LoadParams<Key> {
-    ctor public PagingSource.LoadParams(androidx.paging.LoadType loadType, Key? key, int loadSize, boolean placeholdersEnabled, int pageSize);
-    method public androidx.paging.LoadType component1();
-    method public Key? component2();
-    method public int component3();
-    method public boolean component4();
-    method public int component5();
-    method public androidx.paging.PagingSource.LoadParams<Key> copy(androidx.paging.LoadType loadType, Key? key, int loadSize, boolean placeholdersEnabled, int pageSize);
+  public abstract static sealed class PagingSource.LoadParams<Key> {
+    method public abstract Key? getKey();
+    method public final int getLoadSize();
+    method public final int getPageSize();
+    method public final boolean getPlaceholdersEnabled();
+    property public abstract Key? key;
+  }
+
+  public static final class PagingSource.LoadParams.Append<Key> extends androidx.paging.PagingSource.LoadParams<Key> {
+    ctor public PagingSource.LoadParams.Append(Key key, int loadSize, boolean placeholdersEnabled, int pageSize);
+    method public Key getKey();
+  }
+
+  public static final class PagingSource.LoadParams.Prepend<Key> extends androidx.paging.PagingSource.LoadParams<Key> {
+    ctor public PagingSource.LoadParams.Prepend(Key key, int loadSize, boolean placeholdersEnabled, int pageSize);
+    method public Key getKey();
+  }
+
+  public static final class PagingSource.LoadParams.Refresh<Key> extends androidx.paging.PagingSource.LoadParams<Key> {
+    ctor public PagingSource.LoadParams.Refresh(Key? key, int loadSize, boolean placeholdersEnabled, int pageSize);
     method public Key? getKey();
-    method public int getLoadSize();
-    method public androidx.paging.LoadType getLoadType();
-    method public int getPageSize();
-    method public boolean getPlaceholdersEnabled();
   }
 
   public abstract static sealed class PagingSource.LoadResult<Key, Value> {