Feature flag for which impl. of KeyguardBottomArea

The next CLs introduce a refactor of the internals of
KeyguardBottomAreaView(Controller). This flag is meant to control which
implementation to use in order to limit risk while we work on the
refactor.

Test: Verified, through TheFlippinApp, that the flag exists and that flipping it affects the value read in the SysUI code.
Bug: 235403311
Fixes: 235403311
Change-Id: Ic6c06b047bb5d4b90932dc103a101871f6e4e587