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MIDI ports are now implemented as file descriptors directly between the sender
and receiver, so the MidiService is no longer in the message path.

To facilitate the above, each port has its own file descriptor, rather than multiplexing
all ports on a device through a single socket.

Added a new class MidiDeviceServer, which is used by implementors of MIDI devices.
This replaces the MidiVirtualDevice class (which only was included in changes that were reviewed but never submitted).

The USB MIDI implementation has moved from the MIDI service to the USB service.
The USB MIDI implementation uses MidiDeviceServer as its interface, so we now have a common
interface for all MIDI device implementations.

Change-Id: I8effd1583f344beb6c940c3a24dbf20b477a6436