Enable Back-key when go to Emergency call

When go to "Emergency call" from lock screen, back key is disappeared.
It is difficult for user to find the way to go back to previous screen.
This might be bad user experience. So we are going to make it enable.

Change-Id: Iefcca9b8943472c24ddbca6e3ec3441e93e5be91

Patchset: 2
http://review.cyanogenmod.org/#/c/57524/