DO NOT MERGE Force-enable media feature flag

We won't ever disable it and it's ok to ship it.
Deleting the flag will be done in master given it's a much larger CL.

Bug: 160826507
Test: manual
Test: systemui tests
Test: atest PreferencesHelperTest
Change-Id: I142e7af6316fd9b31cbee2d92df7d89ff42bbc4c