Enable ticker when heads up notifications are disabled

* When heads up notifications are disabled,
  high priority notifications use the old ticker.
* Low priority notification behaviour is unchanged.

Partially based on https://github.com/Euphoria-OS/android_frameworks_base/commit/a41dacb497a6802b88a377134f6c612b7ac6a710

Change-Id: Ie69c42ca1e473876d398e6e3b816e278a5dbed37