OPP: Delete incomplete download file

Disabling Bluetooth during file reception via OPP will cause the file
reception to fail. However, since the receiving process only
terminates after the session is stopped, the incomplete receiving
file is not deleted because mCallback has been cleared when the
session stops.

Bug: 331297926
Bug: 331298337
Test: m com.android.btservices
Test: Manual. Disable Bluetooth during file reception via OPP.
      There should be no incomplete file in Downloads directory.
Change-Id: I66945254c6064958f4a71a28f22ae2d8cebf0deb