Use secure keyguard in validateStackFocusSwitchOnStackEmptied.

This way, the test will be valid regardless of whether unsecured
keyguard is supported or not.

Bug: 192123183
Test: atest android.server.wm.MultiDisplayPolicyTests
Change-Id: I07cbd8f864ff42c5801e46009aaf5f729685a85c
1 file changed