LeAudio: Improve group config strategy recalculation

Currently the group strategy is always recalculated from the PAC record
LTV map entries. This change adds the strategy caching to avoid recalculations
on each GetGroupStrategy call, and potentially allows for other strategy
recalculation methods, not related to AudioChannelCounts LTV type, which
may be absent in the vendor codec PAC records.

- recalculate group configuration strategy only when the cached
  value is invalidated
- invalidate the group configuration strategy only when needed
  (group configuration is changed)
- properly name the helper function (it is Sink only) to avoid
  misunderstanding and misuse

Bug: 308427707
Test: atest bluetooth_le_audio_test bluetooth_le_audio_client_test
Flag: EXEMPT; refactor for the multi-codec feature, verified with unit tests
Change-Id: Ic72f7d84eabf5e68e6abbb946be2e6fb31ca03d7