mediaprovider: add functionfs ioctl

Addresses the following denial:

type=1400 audit(0.0:51894): avc: denied { ioctl } for comm="MtpServer" path="/dev/usb-ffs/mtp/ep1" dev="functionfs" ino=30291 ioctlcmd=0x6782 scontext=u:r:mediaprovider:s0:c512,c768 tcontext=u:object_r:functionfs:s0 tclass=file permissive=0 app=com.android.providers.media

Test: policy compiles.
Change-Id: I5290abb2848e5824669dae4cea829d4cbea98ab4