Toast: fix force close if called from system service

If a toast is created from a system service, an application context
will not be available, use the service context instead.

Fixes
http://code.google.com/p/android/issues/detail?id=40058
http://code.google.com/p/android/issues/detail?id=40075

Change-Id: Ic643f4557fa04476819cd2c3ee640b735eeae429