LeAudio: Fix not releasing the codec resources

After stream gets suspended and the active group goes to IDLE state, we
should be cleaning up the codec. It gets created when the stream is
resumed.

Bug: 295972694
Bug: 262481440
Test: manually - start, stop stream and wait, then resume and observe no warning about the encoder instance not being destroyed
Change-Id: I781e603e0453984841821197c139f2b8927173bf