Remove TouchState::down variable

This variable determines whether touch is currently down. However, we
are already storing the pointers that are down for each window. We can
just check whether any of the touched windows have pointers inside to
replace it.

Bug: 211379801
Test: m inputflinger_tests && adb sync data && adb shell -t /data/nativetest64/inputflinger_tests/inputflinger_tests
Change-Id: I4137e180835da1e689c89feff7c8f223b79aa85e