Move MediaProvider to its own domain, add new MtpServer permissions

Also move necessary priv_app permissions into MediaProvider domain and
remove MediaProvider specific permissions from priv_app.

The new MtpServer permissions fix the following denials:

avc: denied { write } for comm=6D747020666673206F70656E name="ep0" dev="functionfs" ino=12326 scontext=u:r:priv_app:s0:c512,c768 tcontext=u:object_r:functionfs:s0 tclass=file permissive=1

denial from setting property sys.usb.ffs.mtp.ready, context priv_app

Bug: 30976142
Test: Manual, verify permissions are allowed
Change-Id: I4e66c5a8b36be21cdb726b5d00c1ec99c54a4aa4