Track focus changes on external displays (1/4)

Let InputDispatcher can have multiple focused windows (each display
has one). If an event is display-specific, then it will be dispatched
to the focused window in the corresponding display. Otherwise, it will
be dispatched to the focused window in the focused display which is
the display that the user most recently interacted with.

Modified API:
    void setFocusedApplication(
            const sp<InputApplicationHandle>& inputApplicationHandle,
            int32_t displayId)

New API:
    void setFocusedDisplay(int32_t displayId)

Test: atest -a inputflinger_tests
Test: atest ActivityManagerMultiDisplayTests
Test: atest CtsWindowManagerDeviceTestCases
Test: atest com.android.server.wm.DisplayContentTests
Bug: 111361570
Change-Id: I19420deff854c0ee74992569330eaae08e2516e7