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

AndroidManifest.xml

@@ -128,6 +128,8 @@
             android:label="@string/title_preferences"
             android:configChanges="keyboardHidden|orientation|keyboard"/>
 
+        <service android:name=".Service" />
+
         <provider
             android:readPermission="org.thialfihar.android.apg.permission.READ_KEY_DETAILS"
             android:name="org.thialfihar.android.apg.provider.DataProvider"