Revert "Revert "SystemUI: Add emergency call button in lock screen""

This reverts commit 0de64ac1d206a1e8db033385c9c8a27cd6524323.
Keyguard: Add config to always enable emergency call button

As the original design, emergency call button will be hidden on slide
lock screen. Add config_showEmergencyButton to control emergency call
button on slide lock screen.

CRs-Fixed: 640998
Change-Id: Ica29e1ec9043272e9d507307fb8744e8eef57b2e
[1/2] Base: Hide power menu on lockscreen

Adds an option to enable/disable power menu on lockscreen.
policy: only disable power menu on secure lock screen