Support monitor input per display (1/2)

To support monitor input per display, monitor channels would
be stored by displayId, so a copy of input event will dispatch
to the same display if monitor channel exist.

If inputWindowHandle is null and displayId is not ADISPLAY_ID_NONE,
registerInputChannel will treat inputChannel as monitor for displayId.

Bug: b/115602714
Test: atest DisplayContentTests ActivityManagerMultiDisplayTests
Test: atest inputflinger_tests
Change-Id: I1822779cecf6da37b4cd4821149692e3a95add7a