Add capability for OEMs to supply their own SearchIndexableResources.

This moves SearchIndexableResources to be supplied by FeatureFactory
rather than its own singleton, which in effect allows OEMs to supply their
own, in the case where they have their own classes they want to be
indexed (or, remove certain classes that used to be indexed).

Bug: 72179744
Test: All tests pass.

Change-Id: Ia06b2026df7eca4c53b44a5a589c4aaa0b69d96c