Remove edge slop handling from ScaleGestureDetector.

The edge slop code could violate invariants of ScaleGestureDetector,
such as the assumption that if an ACTION_POINTER_DOWN is observed
or if getPointerCount() >= 2, then there must be at least two
active pointers to choose from.  But due to the edge slop handling,
it was possible for findNewActiveIndex to return -1 in this
case, resulting in a crash.

Bug: 6613154
Change-Id: I4e08e38a49ab27dac1be9484e19de086bc43624a