[le audio] Add Volume Control APIs for connected devices in broadcast

This commit includes
1. Add new device volume control API and callback
2. Volume changed callback is invoked for newly registered client
3. Volume change is only updated to audio system if it was applied for
   active devices

Flagged by: leaudio_broadcast_volume_control_for_connected_devices and
enable by adb command
adb shell device_config put bluetooth com.android.bluetooth.flags.leaudio_broadcast_volume_control_for_connected_devices true

Bug: 298661871
Bug: 307408418
Test: atest BluetoothVolumeControlTest VolumeControlServiceTest
Test: manual test callback with LeAudioTestApp
Change-Id: Ibcc35c7a5e9a5cb38ed9b8373d6a128045aae36f