Invert some InputConfig flags to simplify default behavior

Invert NOT_TOUCH_MODAL to TOUCH_MODAL, and SPLIT_TOUCH to
PREVENT_SPLITTING. Modal windows and windows that prevent splitting are
exceptional behaviors, so we make sure that these show up when
outputting flag values using ftl::Flags::string().

Bug: 216806304
Test: atest inputflinger_tests
Change-Id: I7cadcc830f06ff0c63da3b61a1a7580cb031f0c2