Add suggestions to settings

Use an XML to define the categories that Settings will look for,
and surface enabled activities under those categories as suggestions.

When clicked on the activity will be started for result.  If the result
is not cancelled, then the operation is assumed successful and the
suggestion is disabled.  Users can also use an overflow -> remove
flow to get rid of unwanted suggestions.

Change-Id: I767abf8efe103af0509bc6b6b55888ae82643512