Fix bug #15342475 Settings search ranking improvement

- introduce the "base rank" concept. All SearchIndexablesProvider
will have a base rank and the Settings one will get a "0" base rank.
- use a map whose key is the authority name for the provider to get
and store the base rank
- if a base rank is not defined, use the current max defined one and
increment it to define the new one.

Then we will compute the real rank this way:

rank = (providerRank > 0) ? baseRank + providerRank : baseRank

Change-Id: Ifa82bfcdfc07f25c4a5f304f47550686f7d4c13d