Wrap ViewGroup content in save/restore to protect composited children

bug:15570351

Pos Z composited children are drawn before the primary restore for a
RenderNode. This means that without an additional save/restore (which
wraps the content) they aren't protected from transformations/clips
that happen within the ViewGroup's DisplayList.

Also changes RenderNode to use OpenGLRenderer tag for displaylist
dumping consistency, and simplifies the entry points into RenderNode
drawing, since the distinction between tree root vs tree internal is
no longer important.

Change-Id: I2621eba0592c74d71f85a91a4ab3d0da2d7468d3