Simplify unit test of activity launch ability on trusted display

It is better not to mix real world things (virtual display) with
mocked objects. And it is not needed to create a display and
activity record, just focus on Display#isTrusted and the method
isCallerAllowedToLaunchOnDisplay.

Bug: 159102753
Test: ActivityStackSupervisorTests#testDisplayCanLaunchActivities

Change-Id: I54dff649aae4fe894de024637559a74f63b6da25