Move Index provider conversion into Settings provider

Pre-patch, settings search provider would push all of its
fragments into to search via SearchIndexableResources with
an implicit contract of if the resource's xml == 0, then
it was a settings fragment with an Index provider.

One, implicit contract is bad. Two, it was messy at indexing time.

So this patch moves htat conversion into the search index provider.
Such that all of the indexables are either real Resources or Raw.

Change-Id: I39f4351c03d123bb9b45edb4df7f924cfaff2b38
Fixes: 65376542
Fixes: 37741509
Test: robotests