Allow kernel sdcard access for MTP sync.

Address denials such as:
avc: denied { write } for pid=2587 comm="kworker/u:4" path="/storage/emulated/0/Download/AllFileFormatesFromTommy/Test3GP.3gp" dev="fuse" ino=3086052592 scontext=u:r:kernel:s0 tcontext=u:object_r:sdcard_internal:s0 tclass=file permissive=0

Change-Id: I351e84b48f1b5a3361bc680b2ef379961ac2e8ea
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Bug: 15835289