Don't allow reflection on ScaleGestureDetector#mSpanSlop.

This is a value provided by ViewConfiguration that is standardized
across the device.  There are two problems in changing this:
  1. inconsistent behavior with the rest of the uses
  2. modifying this would open the door to modifying all slops,
     which is infeasible and has no good API story

Bug: 123769434
Test: No API or behavior changes
Change-Id: I30bc9d065be8633752a67bd0faa9eafe1678e843