Fix coding style.

Change-Id: I520b16f1c9a927a93d9e1bd4f7e712769d341e58
FLAG_ONE_SHOT is not guaranteed to update the extras.

In contrast, according to the docs [1], FLAG_CANCEL_CURRENT is. Also
keep track of the notifications on a per-device basis.

[1]
http://developer.android.com/reference/android/app/PendingIntent.html

Change-Id: Ie04f60cde6a4332178373ce6be073e04036676c0