Add Indexable.SearchIndexProvider.getNonIndexableKeys(Context)

- getNonIndexableKeys(Context) allow a SearchIndexProvider to tell which data
he does not want to index by providing a list of the data keys
- use this new API for SoundSettings and removing KEY_EMERGENCY_TONE related
settings if the device is not CDMA
- add a BaseSearchIndexProvider for code simplification

Change-Id: I23633ace1d7e390ee05fac0a5458a33e04e72d8d