LeAudio: Minor refactor using BidirectionalPair

- replace other sink/source pair structures with the generic
  BidirectionalPair<>
- replace the legacy Active Context naming with Available Context
- store the BidirectionalPair of supported contexts for group instead of
  one combined value - important the upcoming patches

Bug: 285647765
Test: atest bluetooth_le_audio_test bluetooth_le_audio_client_test
Change-Id: Ic6759c06d060880235b222f20eefd9a9ad0713bb