Partially revert "mediaprovider" SELinux domain.

The new domain wasn't fully tested, and it caused many regressions
on the daily build.  Revert back to using "priv_app" domain until we
can fully test and re-land the new domain.

Temporarily add the USB functionfs capabilities to priv_app domain
to keep remainder of MtpService changes working; 33574909 is tracking
removing that from the priv_app domain.

Test: builds, boots, verified UI and downloads
Bug: 33569176, 33568261, 33574909
Change-Id: I1bd0561d52870df0fe488e59ae8307b89978a9cb