Fixes to HeadsUpManagerPhoneTest & friends

* Fix coverage. Because createAlertingNotificationManager() was not correctly overriden (broken in ag/16852731) the unit tests declared on the base class (AlertingNotificationManagerTest) were _not_ using the appropriate SUT in the descendant test classes.
* Make testExtendHeadsUp() faster by resource-overriding the mExtensionTime with a much shorter value than its actual one of 10 seconds.
* Minor formatting/style issues.

Test: atest AlertingNotificationManagerTest HeadsUpManagerTest HeadsUpManagerPhoneTest
Fixes: 294042016
Change-Id: Ia7fa536b8eb9886c645d06aac85e5d9264ceb86d