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: I2e2637626d9f81780f48625bef52e861180e002c
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>