Add a native Canvas wrapper.

Instead of storing a direct SkCanvas reference, Canvas now tracks
an opaque native wrapper class. The native wrapper can be used to
store additional info for emulating deprecated Skia features
(at this point it only stores a canvas).

Some notes:

* all native handle -> SkCanvas conversions are routed through a
  handful of native utility methods.
* safeCanvasSwap() refactored as a lower level setNativeBitmp() - which
  is what clients need.
* removed unused get_thread_msec() (Canvas.cpp)

Change-Id: I715a5a6f1e1621c1cfc1e510ae4f2ea15cf11114