[flexiglass] Immediately locks device when sleep button is pressed.

Bug: 378708150
Test: manually verified that hitting the sleep button (using "adb shell
input keyevent 223") immediately locks the device even if the "lock
after screen timeout" setting is not set to "immediately"
Test: added a unit test
Test: atest
PlatformScenarioTests:android.platform.test.scenario.sysui.notification.LockscreenSilentNotificationVisibility#goToLockscreen_silentShown_showAlertingNotification
-- --abi arm64-v8a
Flag: com.android.systemui.scene_container

Change-Id: Ia7cc37cf65d9f6b720e56baf1ef9ab84d6331875