Prevent reflection - ScaleGestureDetector#mMinSpan

Also makes "minScalingSpan" accessible to developers
via ViewConfiguration in case developers were accessing
mMinSpan in order to simply read the value of the field,
since that was previously the only way to read the value.

Fixes: 123768938
Test: android.view.cts.ViewConfigurationTest

Change-Id: Ib179aa153f00a55b6913c7497a1e43bd101f41d3