MTP: Respect remove_source in do_push.

I still think it's odd that it's the backend's responsibility to
delete the source file, as opposed to the client.
WIP: Use android extensions to support in place read of files.

Allows for opening files directly on the device and reading from them.
WIP: Use android extensions to support in place write to files.

Allows for opening files directly on the device and writing to them.