MIDI Manager: Remove message packetization from MIDI transport

Instead of attempting to package exactly one MIDI message in each call to
MidiReceiver.onPost(), we now pass messages as a raw stream of bytes.
This means we may now receive multiple MIDI messages in MidiReceiver.onPost().

We make this change to avoid the complexity of taking the single message approach
with SysEx and realtime messages. This shifts some of the burden of parsing
MIDI messages to the client application. But the parsing is best handled in the
application anyway, rather than having the framework impose its own policy
on how the messages are parsed.

Change-Id: Idb6c200037f827cc618bc7d9455a7aa864b494a7