Update transient navigation confirmation wording & behavior

1. Decrease transient navigation confirmation annoyance.

  - Only use the power-key as a signal if we detect a screen-off
    screen-on within a short threshold value.

 - Auto-confirm if user performs the indicated gesture.

 - Remember confirmation across reboots.

2. Update wording to new final wording.  Remove now obsolete
short + long versions.  Decrease message font temporarily
until the new platform toast redesign is finalized.

3. Remove pre-ship ImmersiveModeTesting debug helper.

Bug:10602929
Change-Id: I0bff826391058c7b282eeb61817b93b79de84893