Remove html/* attributes in getClonedElement()

Subbu said that cloning of attributes like data-parsoid or typeof would
cause problems for Parsoid.

Also remove the attributes object if it becomes empty, and do the same
for the internal object.

Bug: 47297
Change-Id: I428becf95c70d0ed8af5b0c408e3966dc47fd8c3
Merge "Remove html/* attributes in getClonedElement()"