[le audio] Improve broadcastAssistant callbacks handling

This commit improved the handling of BroadcastAssistantCallbacks,
it merged assistant callbacks into BroadcastAssistant for better callbacks management
and manage callbacks in the similar way as other Bluetooth profiles. e.g. re-register callbacks on ServiceConnected.
Also it removed unnecessary stack trace log since the client might be non-persistent UI modules.

Bug: 286943323
Test: atest BluetoothLeBroadcastAssistantTest
Test: manually check callbacks

Change-Id: Ica4f4869b2b0992d06b62f45ede94021e5e639af