Add a SysPropBooleanFlag for the lockscreen-lwp project.

This is in order to teamfood the flag and flip it more easily. The flag
is SysProp since accessed from different repos (mainly Frameworks/base and LivePicker)

Bug: 273443374
Test: manual: flip the flag in the flippin app and observe the change
Change-Id: Ic5b488a80b8e946069fd4eadbd895c2bb8e7f4bc
10 files changed