[scenic] skia_view: view_provider->base_view

All topaz clients expose both the V1 and V2 ViewProvider interfaces,
prepping them for the full move to eventpairs.

No dependencies remain on ui/view_framework in topaz examples.

TEST: set_root_view <example> for the various examples

SCN-898 #comment
SCN-993 #comment

Change-Id: I1f02591081b092ad5b040b8126b0dff10b621595
35 files changed