Mark slipWallpaperTouch as const

This function is already not modifying the dispatcher state, so let's
mark it as const. This will allow other functions that depend on it to
also be marked as const in the future.

Other small fixes in this CL:
1. Remove code after fatal log. This code will never execute.
2. Use MotionEvent::getActionMasked

Bug: 273376858
Test: presubmit
Change-Id: I9aaba33f7c3b5dd29a45b120c2823cd41f7f8380