Add panic detection to back button

Adds "panic" detection to the back button.  Implemented solution
uses 4x button presses in a short duration to detect for "panic".
The value used to determine the duration between key up and key down
that still count as a multi-button press is configurable via the
Settings Provider.

BUG: 28027764

Change-Id: Ibf1370ff3cb539a9a54002a8704922744a3ca5d7
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 5da55b1..f4c642f 100755
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -5345,6 +5345,13 @@
         public static final String LONG_PRESS_TIMEOUT = "long_press_timeout";
 
         /**
+         * The duration in milliseconds between the first tap's up event and the second tap's
+         * down event for an interaction to be considered part of the same multi-press.
+         * @hide
+         */
+        public static final String MULTI_PRESS_TIMEOUT = "multi_press_timeout";
+
+        /**
          * List of the enabled print services.
          *
          * N and beyond uses {@link #DISABLED_PRINT_SERVICES}. But this might be used in an upgrade