Instead of using NotificationClient directly from GeckoAppShell, add a
NotificationListener interface, which NotificationClient would
implement. This isolates NotificationClient (and the notification
package) from GeckoAppShell and lets us move the notification package to
Fennec. It also makes a cleaner interface for GeckoView consumers to
implement notification support.
Combine the callObserver and removeObserver methods used for notifying
alert listeners into the native method
GeckoAppShell.notifyAlertListener. Keep track of the listener and the
alert cookie on the native side so that we don't need
GeckoAppShell.ALERT_COOKIES anymore.
Refactor the notification-related methods in GeckoAppShell. In
particular, combine showPersistentAlertNotification into
showAlertNotification, and move code out of AndroidBridge.
