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
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index c1c826c..c8312e3 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -4365,6 +4365,9 @@
         /** @hide */
         public static final String BAR_SERVICE_COMPONENT = "bar_service_component";
 
+        /** @hide */
+        public static final String TRANSIENT_NAV_CONFIRMATIONS = "transient_nav_confirmations";
+
         /**
          * This are the settings to be backed up.
          *