Fix @NonNull -> Nullable for Intent.replaceExtras().

extras == null is documented to result in all extras being
erased, and the implementation handles it correctly. The
@NonNull annotation on Bundle extras is therefore wrong.
This CL replaces it with the correct annotation, @Nullable.

The incorrect annotation was introduced in April 2017
(commit 30e06bb6). I've
looked through the other changes to Intent.java in that
commit but have found no further nullability annotation
errors.

Bug: 121438778
Test: Treehugger
Test: Looked through the other nullability annotations on
      Intent.java introduced by the same commit
      30e06bb6 (Apr 2017)
      but found no further errors.

Change-Id: Iebbe17abc5c97146533e82114fbaf1d7036fd03a