Autofill, Content Capture, and Augmented Autofill API fixes:

- Added @Nullable / @NonNull on some returned values.
- Removed APIs that won't be implemented for Q.
- Made Builders final.

Test: m update-api

Bug: 123100695
Fixes: 126702168
Fixes: 126701052
Fixes: 126702191
Fixes: 126700602
Fixes: 126700135
Fixes: 126700413
Fixes: 126698817
Fixes: 126698792

Change-Id: I20710537c02051361da18370bc5cc642cc3c26e1