Enforce non-null in BoundaryCallback (lost in kotlin convert)

Test: ./gradlew paging:p-c:test paging:p-run:cC paging:p-rx:cC

Change-Id: I90d4c4f5d2d165ec186fe4552947761619a3eca0

diff --git a/paging/common/api/current.txt b/paging/common/api/current.txt
index ed2cabf..76e57f9 100644
--- a/paging/common/api/current.txt
+++ b/paging/common/api/current.txt

@@ -192,8 +192,8 @@
 
   @MainThread public abstract static class PagedList.BoundaryCallback<T> {
     ctor public PagedList.BoundaryCallback();
-    method public void onItemAtEndLoaded(T? itemAtEnd);
-    method public void onItemAtFrontLoaded(T? itemAtFront);
+    method public void onItemAtEndLoaded(T itemAtEnd);
+    method public void onItemAtFrontLoaded(T itemAtFront);
     method public void onZeroItemsLoaded();
   }