Add nullability annotations for current behavior.

No behavior changes.  This paves the way for a future CL that is
will be adding a handful of new APIs.

Bug: 131598520
Test: atest android.content.cts.ContentProviderOperationTest
Test: atest android.content.cts.ContentProviderResultTest
Change-Id: I1a6f53e79009ba88b13844a6c8ca8c09e33ee5b2
3 files changed