Remove read access from mls constraints.

Addresses the following denial encountered when sharing photos between personal
and managed profiles:

Binder_5: type=1400 audit(0.0:236): avc: denied { read } for path="/data/data/com.google.android.apps.plus/cache/media/3/3bbca5f1bcfa7f1-a-nw" dev="dm-0" ino=467800 scontext=u:r:untrusted_app:s0:c529,c768 tcontext=u:object_r:app_data_file:s0:c512,c768 tclass=file permissive=0

Bug: 19540297
Change-Id: If51108ec5820ca40e066d5ca3e527c7a0f03eca5