[flexiglass] Support for delayed locking

Depending on user settings like Screen Timeout, Lock after screen
timeout, and Power button instantly locks - locking the device after
it's put to sleep might or might not be instant. This CL adds support
for this.

Fix: 376759587
Test: manually tested a screen timeout of 15 seconds with a lock after
screen timeout of an additional 15 seconds - made sure that turning on
the screen within 15 seconds stays on the Gone scene
Test: manually tested a screen timeout of 15 seconds with a lock after
screen timeout of an additional 15 seconds - made sure that letting the
timeout occur correctly locks the device and that turning it back on
again reveals the Lockscreen scene
Test: manually tested that setting "power button instantly locks" means
just that
Test: manully tested with a non-secure auth method, even if there is a
previously-set non zero value for 'lock after screen timeout', it still
instantly locks
Test: unit tests added
Flag: com.android.systemui.scene_container

Change-Id: Idd2036d745dc2521bc080151e0a720a20b57c06e