Fix issue #64224738: Document return value of IBinder.transact()

Also add appropriate @NonNull and @Nullable annotations.

Test: built

Change-Id: I22de48105ef685baf594cfc004dd3e27e2ba09e9
Merged-In: I22de48105ef685baf594cfc004dd3e27e2ba09e9
(cherry picked from commit 4cd650c0085e6dd20d3f46c5b668e54537f887cf)
2 files changed