Add new facility to find out when a PendingIntent is canceled.

This is just an internal API in the platform, not (yet?) available
in the SDK.  But it will be useful for system services that want to
clean up state if a pending intent that has been registered with them
is canceled (either explicitly by the app, through the app being
uninstalled, etc).

Also improve the activity manager's dump of pending intents to
organize them by package, making it much easier to read (now that
we have so many active pending intents these days).

Test: ran and booted.  no CTS, since no API.

Change-Id: Iad029cfedcd77e87357eca7da1b6ae94451dd981