USB MIDI: Create encoder per output port

UsbMidiEncoder is not thread-safe. Therefore, each output port
should use its own encoder.

Bug: 225884009
Test: MidiKeyboard/MidiScope on both MIDI 2.0 and MIDI 1.0 paths.
Test: Verified M-Audio Keyboard with only input has the correct logs.
Change-Id: I43bd7381982403766b7f37c22d55ef5d19013120