[flexiglass] Adds support for "lock now" from KeyguardService

Bug: 378708150
Test: manually triggered the code path using "adb shell cmd
accessibility call-system-action 8" which triggers the call to
KeyguardService.doKeyguardTimeout and verified that it locks
immediately regardless of timeout
Test: added unit test case
Flag: com.android.systemui.scene_container

Change-Id: I782370024af222b24801fd23b03c89ba418f6a25