Fix bluetooth crash when no bluetooth.audio provider is present

The stack does fall back to IBluetoothAudioOffload, if present, rather
than crashing.
This is observed on some GSI builds.

Bug: 179967994
Test: start Bluetooth on device without bluetooth.audio HAL
Change-Id: I2979d43a1721f5d41b3a89ec14f670b16ef0d660