Add Geocorder.isImplemented()

The Geocorder interface is not part of the Android core.  It
requires a backend service which may or may not be available
on a device.  The new isImplemented static method allows
apps to discover whether the Geocorder is in fact available
on the device.

Change-Id: I2b5cf7bcc9cce4766bcbb156e91edf34b01f9296