Support nullability annotations in core platform APIs

Previously, adding nullability annotations to methods that were part of
either the legacy or stable core platform APIs would have no effect on
the generated stubs of the .txt files. This change rectifies that.

At the moment there are no methods in the core platform APIs that have
nullability annotations. Change https://r.android.com/1657840 will add
the first ones.

Bug: 183938110
Test: make update-api - after cherry picking https://r.android.com/165784
Change-Id: Idd9f4f38a995bc31a0f1a5702c1617d2de6ccc42
2 files changed