Grant apps write access to returned vfat FDs.

Users can pick files from vfat devices through the Storage Access
Framework, which are returned through ParcelFileDescriptors.  Grant
apps write access to those files.  (Direct access to the files on
disk is still controlled through normal filesystem permissions.)

avc: denied { write } for pid=3235 comm="Binder_1" path=2F6D6E742F6D656469615F72772F373243322D303446392F6D656F772F6D79206469722F706963322E706E67 dev="sdb1" ino=87 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:vfat:s0 tclass=file

Bug: 19993667
Change-Id: I24b4d8826f0a35825b2abc63d1cfe851e1c1bfe9