[flexiglass] Move "show lockscreen when folded" to the main thread.

As of ag/27342795, changing the scene on STL throws an exception when
called off the main thread. This CL fixes it for the
KeyguardService.showDismissibleKeyguard case.

Test: manually verified that the scene is changed to lockscreen when
folding closed an unfolded, unlocked foldable
Bug: 341445974
Flag: com.android.systemui.scene_container

Change-Id: I2787ccc8fe94caeabdc709d74e5665f4f0400e19