Floss: Support multiple volume callbacks

Currently we assume there is only one volume callback and we always
replace the callback if a new audio device is connected.

When two audio devices are connected simultaneously, the later device
is disconnected because we can only stream audio to one device.
However, the volume callback is tied to that device, so now if the user
changes the volume, we can't call the correct callback.

This CL stores callback to all connected devices, so we can get the
correct callback when user updates the system volume.
Also, to simplify things, whenever floss layer receives an AVRCP
connection when there is an existing connection, we ask libbluetooth
to drop the newer connection. This behavior can be updated later, e.g.
when the old connection is disconnected we can smartly switch to the
new connection.

Bug: 366387141
Test: m -j
Test: Pair two headsets that connects AVRCP when they reconnects.
      Connect each headset one by one. Verify the 2nd device is
      disconnected, and the 1st device still receive volume updates.
Flag: EXEMPT, floss only change

Change-Id: I2e0f707fd11a3a6dbf827d839f95ec57f47e98d9