Increase leniency of precondition check for MotionEvent::split

The severity of a specific precondition check was unintentionally
promoted from WARNING to FATAL in the following refactor:
I6230b6aa0696dcfc275a5a14ab4af3d4b7bd0b45

Here, we demote it back to a non-fatal check.

Bug: 328852741
Bug: 329107108
Test: atest inputflinger_tests
Change-Id: If2cb22d528d5e68c1e035a3e4291dae7fc32bb34