base

MidiManager: Add MIDI device status notifications

MidiManager clients can be notified of device status changes via a new MidiDeviceStatus object.
MidiDeviceStatus contains the busy status of the device's input ports and number of
connections to the output ports.
MidiDeviceService now has an optional callback for receiving notifications when its ports change as well.

Change-Id: I1600df4464d82724bc026c27b9633ae9c412d3f0