Generate a new id for each window

When input receives a list of windows from surfaceflinger, each window
has a unique id. But when we run tests, all ids are the same. This
causes some unexpected test failures for the cases where we have more
than 1 window per display.

To avoid this issue, mirror the surfaceflinger approach of generating
the ids by keeping a static variable that gets incremented every time we
create a new window

Bug: 143459140
Test: atest inputflinger_tests
Change-Id: I36731e78e16892b4bf48d6eba8db3e4c2684b54d