Dismiss staged split when keyguard is occluded

Dismiss staged split if there's show-when-locked activity showing on top
of keyguard. Add a callback to monitor keyguard occluded state and
delegate it to StageController in WMShell, so that it could dismiss
staged split properly when keyguard is occluded.

Bug: 188010345
Fix: 187360909
Test: WMShellUnitTests
Test: launch show-when-locked activity while keyguard showing and staged
split activated, observed the staged split will be dismissed.

Change-Id: If93b36f3fcef969d449c505086459997c27b8c75