Clean up captions service checks

This CL finish the migration to the new CaptioningManager API.
As part of the cleanup the captions service config is also removed,
and the PackageManager implementation is updated to take the captions
package name from the remaining config value which is stil in use.

Change-Id: Icba274c904c58f62c1036deac936114b7deb8553
Bug: 220968335.
Test: Manually.