Fix and clarify behavior of vertical/horizontal attributes

There were a couple of problems with the new paddingHorizontal
and layout_mareginHorizontal attributes. For one thing, the behavior
of layout_marginHorizontal needed to change with respect to marginStart/End.
Instead of the implemented behavior where Horizontal took precedence over
start/end, the behavior is being changed such that start/end can override
horizontal. This makes it consistent with the way that the attributes work
for padding.

Also, the attribute docs were not correct. For one thing, they needed to be
updated to match the new behavior for marginHorizontal. Also, the docs for
the padding attributes (including the docs for the existing "padding") were
not correct for the behavior as-implemented (specifically with respect to the
precedence of the attributes where paddingStart/End are concerned).

Bug: 37756178  double-check logic of horizontal/vertical attributes wrt start/end attributes
Test: Updated cts tests, submitting at the same time
Change-Id: I85a102549022cbec7d7b5c76f31ac985db103372