Make singleTop activity be created with different display

If an instance of the activity already exists at the top of the
current task, the system routes the intent to that instance,
rather than creating a new instance of the activity. The top
task is examined from the global focused stack.

This CL checks the existing singleTop instance just from the
launching display.

Bug: 175098384
Test: atest ActivityStarterTests
      atest IntentTests
      atest MultiDisplayActivityLaunchTests
Change-Id: Ia0ffe6b8de6bcae738e273b998fd275e70d741cc