Add layout_marginHorizontal|Vertical attributes

These two new attributes are a short-hand way for specifying the
separate left/right (or start/end) and top/bottom margin attributes.
It is common to use the same margin on both edges, so these new
attributes make that common case easier and less verbose.

Issue: 30261490 Better way to define paddingTop/Bottom/Left/Right in XML if vertical paddings are the same and horizontal paddings are the same
Test: new CTS test in ViewGroup_MarginLayoutParamsTest
Change-Id: I183ace956892187b6ea086e61ed5b195c8903d7d