Add support for Multi-A2DP state machines per device

* Update usage of A2dpService API calls that take BluetoothDevice
  as an additional argument
* Update the description for BluetoothA2dp.connect()

Exempt-From-Owner-Approval: De-facto owner of the relevant changes is
the Bluetooth team.
Bug: 69269748
Test: Manual
Change-Id: I190ed48ef65bbc8b88b45f84ebd6ab3d21cf0b4e