Activate connected a2dp devices via AudioRoutingManager

Both AudioRoutingManager and A2dpService try to activate the connected
a2dp devices. This CL remove the logic in A2dpService so that
AudioROutingManager would be the centralized module for handling device
activations.

Bug: 299023147
Test: atest BluetoothInstrumentationTests
Change-Id: Ic0c6914cbf1f1da4b06b7d567e7bf37c65cc9035