OS: Android

We should ensure that the browser process cannot be killed (eg for low memory reasons) while a notification interaction event is being processed.

For the NotificationJobService code path (see  http://crbug.com/663427 ), we already hold this wakelock while starting up the browser, but there is still a window where our process may be killed whilst the notification is being processed by async native code which calls the service worker (which is allowed up to 1 minute of execution time).

We can retain the wakelock Android gives us in onStartJob by by returning true there, and then calling jobFinished once the event has been completely handled (ie once serviceWorkerRegistration.onNotificationClicked has returned).