MidiManager: proxy all requests to open devices through MidiService

Previously when a MidiManager client opened a virtual or Bluetooth device,
the client bound directly to the virtual device's MidiDeviceService
or BluetoothMidiDevice's IMidiDeviceServer for the given BluetoothDevice.
Only USB devices were opened in MidiService.

Now opening any type of MIDI device is done via IMidiManager.openDevice() or
IMidiManager.openBluetoothDevice().  MidiService tracks all connnections between
clients and devices.

Services that implement virtual devices must now require android.permission.BIND_MIDI_DEVICE_SERVICE
so only MidiService can bind to these services.

Bug: 21044677

Change-Id: I7172f7b1e0cbfe4a2a87dff376c32dc9b41aa563