Add temp flag for lockscreen/aod ui prototyping

Add new Settings.Global.SHOW_NEW_LOCKSCREEN property. When enabled, new
lockscreen & aod UI + transitions will show. This is a temporary flag for the
purposes of protoyping which will be removed before release. Therefore, it should
not be backed up.

To toggle the flag (value = 0 is disabled, value = 1 is enabled):
adb shell settings put global show_new_lockscreen <value>

Test: manual
Bug: 170228350
Change-Id: I3b40b1de9bb80d43218fa1e8661fba47b50e9888