Add sub-elements for "Variant" type

The Quirk, Attribute, Alias, Limit and Feature elements are added to
"Variant" type for fixing VTS error.

Bug: 150833859
Test: m -j40 vts && vts-tradefed run vts -m VtsValidateMediaCodecs
Change-Id: I55066cc1de8b820c24faccdfb469377c897e9808
Merged-In: I55066cc1de8b820c24faccdfb469377c897e9808
(cherry picked from commit cf9526cf)