hotword: Fix inconsistent @NonNull and API comment

- setMediaSyncEvent does not need to accept any @Nullable param.
- getMediaSyncEvent return @Nullable.

This CL manually define the getter to allow for the @Nullable/@NotNull
and doc asymmetry.

Bug: 186469575
Fix: 186469575
Test: CtsVoiceInteractionTestCases
Change-Id: Icc0b6282e6ee13c8fcaa7dea763a39c430e66644