Gallery2: Fix showOnMap method

If the context is ProxyLauncher, showOnMap will make crash the Gallery app if
GoogleMaps not exists, because the double call to startActivity.
Just query the intent before call the startActivity method.

Change-Id: I55472640e2073dc17a1e2c1630215328ae5effc7
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>