le_audio/client_audio: Fix deadlock on Stop()

During the Stop() call, `send_audio()` may still be triggered from
the other thread. Stop() call locks the mutex and waits for the periodic
task to end, while the periodic task might already be waiting for the same
mutex to be released so that it could sink the audio samples with
OnAudioDataReady() callback.

Bug: 150670922
Tag: #feature
Test: atest --host bluetooth_le_audio_test
Sponsor: jpawlowski@
Change-Id: If3653077bb11bd3e4c7034ce727597c9e7250b6f