Unit tests for PowerNotificationWarnings

Just a sample - this is a far cry from full coverage. Mostly
exercises the handoff to the NotificationManager. To go further,
I would verify more of the Notification contents.

Modified the constructor to explicitly take a NotificationManager,
mocking out the Context wasn't feasible here.

Change-Id: Ic83ef17f061a20efc52b7e3e8c022afab2deacdb