leaudio: Fix crash on Sink Resume when group is inactive

This is regression after

c2f9bbd2 le_audio: Introduce Sink listening mode

Normally when group is inactive CancelLocalAudioSinkStreamingRequest()
cannot be called unless Sink listen mode is enabled.

Bug: 315070686
Test: atest bluetooth_le_audio_client_test
Tag: feature

Flag: EXEMPT bugfix actually fixing flagged code delivered in the mentioned regression code. sink_monitor_mode_ is set only when flag is on.

Change-Id: I27dc3eda16df4ca1b8286f224a433270156967f3