Move locale from LanguageDetectionEvent to base class

A follow up change of ag/7058048.

Also fixed a wrong nullability annotation, ConversationActions.getHints
should be @NonNull.

Bug: 129344540
Test: atest cts/tests/tests/view/src/android/view/textclassifier/cts/

Change-Id: Iae734b4b27dd3dd8294684330bec198072693a57