Remove caption when keyguard is visible

Adds a DesktopModeKeyguardChangeListener to record if keyguard is
visible and occluded. If it is, it means that the task is on top of the
keyguard and caption is not usable and should be removed.

Bug: 288910972
Test: Open calculator app and turn the screen off and then on. Caption
should not be there

Change-Id: I199f8233cf55d574bf8d045cb95a710008299b06