Use floor to calculate the range to avoid the rounding error triggered

Use floor to calculate the range to avoid the rounding error triggered
scrollbar. The worst case is that you can never scroll down to the last
pixel. The old special case can cause problem when getViewWidth/Height
have changed which do not match mLastXXSent any more.

Remove FIND_HEIGHT from computeVerticalScrollRange per conversation with Leon.

Change computeMaxScrollY to use computeVerticalScrollRange which matches
how it computes X.

Change onMeasure to use contentToViewDimension() as it has the same logic.