MtpProvider: Implement the delete() method for deleting objects.

Change-Id: I0908f304464f22b156bda89b6d37b6f8fef1d436
Signed-off-by: Mike Lockwood <lockwood@android.com>
Merge commit 'e5fe994a2c8269f9db375ff07a48344c2152c1f8' into HEAD

* commit 'e5fe994a2c8269f9db375ff07a48344c2152c1f8':
  MtpProvider: Implement the delete() method for deleting objects.
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Change-Id: Ibbebce1d6c95472b07ae92d3355e5a60b5b7b45c