Ongoing notification for GPS use.

This change improves upon the notification priority API
introduced in change I9e738cc4, allowing privileged clients
to set the priority of a notification when posting it
directly to INotificationManager. StatusBarTest is updated
to test this new feature.

The new LocationController in SystemUI uses this facility to
post a high-priority ongoing notification whenever GPS is in
use (replacing the functionality of the legacy GPS status
bar icon).

Also happens to fix http://b/3325472 (adding a log message
when notifications are dropped because of a missing icon).

Bug: 3412807

Change-Id: I523016ffa53bf979be98ddc4a2deb55a6270c68a