Track focus changes on external displays (2/4)

Let each DisplayContent has its own focused window and focused app.
This change also moves the last tapped display to the top.

Test: atest ActivityManagerMultiDisplayTests
            ActivityStackSupervisorTests
            ActivityStackTests
            CtsWindowManagerDeviceTestCases
            DisplayContentTests
            PointerCaptureTest
Bug: 111361570
Change-Id: I776cabaeaf41ff4240f504fb1430d3e40892023d