Add remote volume slider conditionally.

Issue before this CL:
RemoteVolumePreferenceController only return available when
it is casting, otherwise return CONDITIONALLY_UNAVAILABLE.
However slice database only index available controllers and keep
this cache. So remote slider won't be indexed if it is not casting
at that time.

As a tmp fix, this CL make controller always return available
unsearchable to make it indexed by database. However only add
that slice if it is casting.

Bug: 130124950
Test: RunSettingsRoboTests
Change-Id: I191144844d6ba7ccbe3dc1c9d19801adb978abc6