[flexiglass] Only animate lockscreen shortcuts when changing doze.

The CL adds logic to make sure that lockscreen shortcuts don't animate
when a Flexiglass scene transition is in progress.

Fix: 297403783
Test: unit tests added
Test: manually verified that switching scenes in Flexiglass does not
cause a vertical translate animation for the lockscreen shortcuts when
returning to the lockscreen scene
Test: manually verified that toggling AOD/doze on and off does animate
in Flexiglass and pre-Flexiglass

Change-Id: I8cdf4d5f038d4f7c4feb5c0e5c3f6e46f7925acf