Remove the Settings search keywords from Slices

The keywords used for settings search are good when we are highly
confident the user is searching for as setting (settings search),
but not effective in a more general search setting (launcher,
an assistant). Thus, we should not index these keywords as Slice
keywords, and rely on the setting title and screen title as baseline
keywords.

Change-Id: I99e44834454b5949c4883f877e02be47498e06e2
Fixes: 78911847
Test: robotests