Allow vold to move FUSE backing files directly.

This enables an optimization of bypassing the FUSE overhead when
migrating emulated storage between volumes.

avc: denied { write } for path="/mnt/expand/6cba9b95-4fc8-4096-b51f-bdb2c007d059/media/obb/.nomedia" dev="dm-0" ino=387843 scontext=u:r:vold:s0 tcontext=u:object_r:media_rw_data_file:s0 tclass=file permissive=1

Bug: 19993667
Change-Id: I2bb9aaca50ed988ded6afec6d7fbe190903707e0