Add bluetooth_media interface

- Adds the initial dBus template for Bluetooth media
- Connects methods to A2DP topshim

Bug: 189497374
Tag: #floss
Test: dbus-send --system --print-reply \
	--dest=org.chromium.bluetooth \
	/org/chromium/bluetooth/media \
	org.chromium.bluetooth.BluetoothMedia.Initialize

Change-Id: Id8fa7675d9867fa156722c939cba5fc95fb28a78