Use isLocationProviderEnabled to test for enabled providers

LocationManagerService was just checking if the string of (comma-separated)
Location Providers contained the provider we were interested in.  This works
fine in normal cases, but breaks if we add a provider such as test_network.
Enabling test_network causes LocationManagerService to think that the network
provider is also enabled.

The code in Settings.Secure.isLocationProviderEnabled() checks for the commas
in the string as well, to make sure that a provider name which is a substring
of another provider name won't cause problems.  It also centralizes the code
which reads the string.

Signed-off-by: Brad Larson <brad.larson@garmin.com>