Enable HUN for Timer notifications.

Bug: 15470349
Bug: 22120450

When the app is open, hide the HUN. Clicking on the HUN opens
TimerFullScreenAlert.
This implementation also fixes the issue where the times-up
notification is not dismissed when timer is dismissed from wear.

Change-Id: Ic264cee5f381692d787a06db191af55a0aed37eb