Adding search for dynamic accessibility settings.

The language and input settings are highly dynamic and this change adds search
support for that. This category depends on installed IMEs, input devices, user
dictionary configuration, etc. We not only compute the right preferences to be
indexed but also track related system state in the settings app to rebuild the
index if needed.

bug:14066763

Change-Id: Ia89d9e35bd79abf8d74614691aedf4ca9b11b6f2