added a service to handle the caching, this'll ensure the cache works while no activity is around, which is better for k9mail integration

it also is a more efficient and much smarter cache, not requiring an own timer thread, just a service that sleeps must of the time, it also is more accurate in cleaning up the entries, ensuring that the worst case of too late removal is 5 seconds

src/org/thialfihar/android/apg/PreferencesActivity.java

@@ -50,7 +50,6 @@ public class PreferencesActivity extends BaseActivity {
                 new Choice(180, getString(R.string.choice_3mins)),
                 new Choice(300, getString(R.string.choice_5mins)),
                 new Choice(600, getString(R.string.choice_10mins)),
-                new Choice(0, getString(R.string.choice_untilQuit)),
         };
         ArrayAdapter<Choice> adapter =
                 new ArrayAdapter<Choice>(this, android.R.layout.simple_spinner_item, choices);