Add docs for new padding/margin attributes.

The attributes for View, paddingHorizontal and paddingVertical,
were added in the O release and are documented in R.attr. But they
should also be referenced in View itself, alongside the other
padding parameters.

Similarly, the new layout_MarginHorizontal and
layout_marginVertical should be referenced in
ViewGroup.MarginLayoutParams.

Bug: 63128350  Add docs about new padding/margin params
Test: built docs, checked the result
Change-Id: I3021df5ea83c469811b4a6ec6ecd3ab2966ec384