Always split touches

In this CL, we are force all windows to use FLAG_SPLIT_TOUCH. This is
needed to avoid complex interactions between this flag and the spy
windows functionality. The old behaviour is likely mostly a historical
artifact, and was not originally intended. With the features like
freeform windows, split screen, and spy windows now possible, we should
always split touch to also avoid potential security issues about
receiving touches outside of your window.

If the functionality of rejecting partial gestures is indeed necessary,
then it will be introduced via a new flag that's only usable by
privileged windows.

Bug: 239934827
Test: atest inputflinger_tests
Flag: com.android.input.flags.split_all_touches
Change-Id: I579651e1309012a6db96730548c12f032c43d88a