LeAudio: Check for start request mutext before getting value

It's possible that resumed stream from StartRequestV2 context may
trigger response e.g. Cancel/Confirm before going out of this method.
Cancel or Confirm callback function relies on start request state which
is guarded by start_request_state_mutex_. Getting state should be also
guarded to avoid races in this case.

Tag: #Bug
Bug: 329179352
Bug: 332847150
Test: mmm packages/modules/Bluetooth
Test: manual streaming regression tests
Change-Id: Ie10d5710329ee60213dd8ed53512d76df1702975