Don't throw an exception from isProviderEnabled and getLastKnownLocation

if the location provider does not exist.  Instead use the same behavior
as if the provider were disabled in settings
(return false for isProviderEnabled and null from getLastKnownLocation).
This eliminates for a lot of exception handling around some simple
queries to the location manager.

BUG: 2841014

Change-Id: I4fbe0c088e915c90969e13083201dd3e7f4029cb
Signed-off-by: Mike Lockwood <lockwood@android.com>