LeAudio: Align context mixing policy

- bidirectional contexts are not mixed with each other
- bidirectional contexts from source metadata are not mixed with the old
  sink metadata contexts
- CONVERSATIONAL from the In Call preference callback call is not mixed
  with contexts from the metadata updates

Bug: 281686787
Bug: 285647765
Test: atest bluetooth_le_audio_test bluetooth_le_audio_client_test
Change-Id: Ib35accf37709b5aa23b92aa99e2cf5d0f737f397