Add NPE check on mBluetoothLeAudioService callback registration

The LE audio service could be unavailable by the time we try to register the callback (the operation is performed under an executor). Add a check to make sure it is available before registering callbacks.

Bug: 387515338
Change-Id: I33d6010dceee5bd2fc8f8f7a7ba0fa13d954237c
Flag: EXEMPT bugfix
Test: Manual