Classify FallbackAssistant Audio as USAGE_ASSISTANT

Since FallbackAssistant acts as an Assistant, its volume
should be in the same audio channel as Google Assistant.
This means the same volume slider that modifies GA's
volume should also modify FallbackAssistant's.

Change-Id: I4fe06e509271f4fc8191c0976118831d47974136
Fix: 134394199
Test: Manual
1 file changed