Post KeyguardGone request to main Handler so multidisplay keyguards are

dismissed properly.

While keyguard is dismissed, onDisplayChanged is called, which calls
hiding and showing presentations. Dismissing keyguard on these
displays should be done when onDisplayChanged is done processing.

Test: Manual
Bug: 156117648
Change-Id: Ib6d2ce407aef2fee22cfec227d243650b5643c25