Basic handling of Notifications with fullScreenIntent.

If a Notification has a non-null fullScreenIntent AND the
topmost Activity is not immersive, that PendingIntent will
fire (presumably causing a nice dialog or full-screen
activity to appear).

Immersive mode handling for FLAG_HIGH_PRIORITY Notifications
is still unimplemented (although the fullScreenIntent will
be suppressed in immersive mode).

Note that currently your fullScreenIntent notification will
also get posted to the status bar, so you're responsible for
clearing it out (e.g. in onPause in your alert intent). See
forthcoming changes to StatusBarTest for an example.

Change-Id: Ie750d1b7bcc788bd29ee1d8626f971dd47fd2817