Rename addHeader -> insertHeaderItem

Intent is:

 - to clarify that these are inside the list, so
   withLoadStateHeader/Footer will be around them

 - to make the methods more discoverable/similar to insertSeparators

 - to clarify these are list items, not dynamic loadstate headers/footers

Test: ./gradle paging:p-c:test
Change-Id: I0675d4b453901a517573c36c96188380512199a5

diff --git a/paging/common/api/current.txt b/paging/common/api/current.txt
index ddd6c00..0b31490 100644
--- a/paging/common/api/current.txt
+++ b/paging/common/api/current.txt

@@ -248,11 +248,11 @@
   }
 
   public final class PagingData<T> {
-    method @CheckResult public androidx.paging.PagingData<T> addFooter(T item);
-    method @CheckResult public androidx.paging.PagingData<T> addHeader(T item);
     method public static <T> androidx.paging.PagingData<T> empty();
     method @CheckResult public androidx.paging.PagingData<T> filter(kotlin.jvm.functions.Function1<? super T,java.lang.Boolean> predicate);
     method @CheckResult public <R> androidx.paging.PagingData<R> flatMap(kotlin.jvm.functions.Function1<? super T,? extends java.lang.Iterable<? extends R>> transform);
+    method @CheckResult public androidx.paging.PagingData<T> insertFooterItem(T item);
+    method @CheckResult public androidx.paging.PagingData<T> insertHeaderItem(T item);
     method @CheckResult public static <T extends R, R> androidx.paging.PagingData<R> insertSeparators(androidx.paging.PagingData<T> pagingData, kotlin.jvm.functions.Function2<? super T,? super T,? extends R> generator);
     method @CheckResult public <R> androidx.paging.PagingData<R> map(kotlin.jvm.functions.Function1<? super T,? extends R> transform);
     field public static final androidx.paging.PagingData.Companion! Companion;