Add test for non-splittable windows

We currently have some code to block incomplete gestures from being sent
to the windows that don't set FLAG_SPLIT (setPreventSplitting to true).
However, this code in untested today.

Add some logs to detect this condition at runtime, and a test to ensure
that the corresponding code isn't deleted.

Bug: 291809671
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: Idc3e9d5b94f67a5700652df35e8983a2a23de0a2