Framework: Disable Heads up in Lock screen

We already have Lock screen notifications

Change-Id: I257b962b67bb25bc9ce5be76144e2c866d7568cf
Heads up: Update snooze drawable:

- Use a flat background, icon and "text"
- Use the background color from notifications for the snooze background, too,
  (especially the pressed background, no big difference for normal background)
- Make the drawables larger,
  (in my opinion the drawables are a bit to small, so use a smaller transparent "border")

Change-Id: I550d04ef618530412e9c8b3927fd0a266277e497