LeAudio: Improve group activation

It is reasonable to configure Le Audio group for the most
recent audio usage. This is most important when groups are
activated during the ongoing streams.

Tag: #feature
Bug: 269394021
Test: atest --host bluetooth_le_audio_client_test bluetooth_le_audio_test --no-bazel-mode
Change-Id: Idaf3ed1507820dbb9f53e8e52076167f7d574654