Change isStylusEvent behaviour

This function is now getting moved to Input.h.

We are also making its behaviour deterministic: if at least one pointer
is stylus, then the event is a stylus event (unless the source wasn't
set to "stylus").

This way, an event with 1 finger and 1 stylus pointers will have the
same consistent behaviour, regardless of the order that the pointers are
stored inside the event.

Bug: 211379801
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: Ie164e32b7c0e9cf21b3819b01a03ac2885666191