Revert "Add panic detection to back button"

This reverts commit 96e9cc57.

The feature was added for Android Wear, however it never got used.
This change reverts the change that added this feature. CTS test for it
has been removed in ag/3340085.

Bug: 62069912

Change-Id: I4f2aa580ebb5fdff236df76eb89d9cef640c1e32