Introduce new preference indicating user control.

If the user enables this, then that means that we will respect user's
choices for all the preferences, i.e. we will not update the preferences
based on the updates from server.
On the other hand, if it's disabled, then the rest of the preferences
would not be considered.  And we'd decide their values based on the
values from the DeviceConfig (which in turn come from the server).

Also, renaming the set prop key for enabling-transcode to match the one
in DeviceConfig - transcode_enabled.

Also, the "transcode_enable_all" controller now enables (instead of
disabling) the global level transcoding.

BUG:169327180

Test: manual testing.

Change-Id: I183b3b5496b647943be377c02c8c9e2a0df71752