Add getDescendants to Settings Slice Provider

getDescendants allows another app to collect a list
of all valid Uris for Settings Slices. Important for search,
assistant or launcher apps.

This means searching our database for all valid keys and building
a list of Uris for each of those keys.

Fixes: 77808328
Test: robotests
Change-Id: I3ae27e4661a7dcaab50b091ae2730013118af8a2