[SettingsProvider] properly clear stale GenerationTracker for Settings.Config

Previously we never cleared outdated GenerationTracker and never fetched
a new one. Subsequent calls have always been IPC which is inefficient.
It doesn't affect in-SystemServer calls but it can be costly for
non-SystemServer clients.

BUG: 228619157
BUG: 277448672
Test: atest Settings_ConfigTest

Change-Id: I55b325a28ce7e0cfa69a6b1b6d70adea86bd87eb