Add test to call setInputWindows twice

Currently, I'm observing some strange behaviour, where calling
setInputWindows twice results in the touchable region becoming empty.
Add this test to R to see what's going on, and potentially bisect on
this.

Bug: 143459140
Test: atest inputflinger_tests
Change-Id: Ia0acef5d4ee4acc29d20174fe44c9f94172ccd96