le_audio: Add safe binds for LE Audio HAL clients

This replaces unsafe calls of base::Unretain with weak pointer.
This solution guarantees to terminate safely if the lifetime
expectations are violated.

Tag: #feature
Bug: 299238288
Test: atest bluetooth_le_audio_test
Change-Id: Ifeb1c252c8c0de4c942cb96ecf3ac456281327a3