MTP host: Add support for reading files from an MTP device via ParcelFileDescriptor

Also added some support for sending files to the device that hasn't been debugged yet.
Add locking to MtpDevice to prevent it from attempting multiple transactions simultaneously.

Change-Id: I2b995ba0af086cc6920bd6b8c869f540ad78560a
Signed-off-by: Mike Lockwood <lockwood@android.com>