Move things in InputWindowInfo to enum classes

Also introduce a new Flag class to make it possible to deal with flags
while treating the individual flags as part of an enum class in a type
safe manner.

Bug: 160010896
Test: atest inputflinger_tests, atest libinput_tests
Change-Id: I915a1c1e3b31f1c0fd99b83ba5fad7e537cd6f84