Separate Canvas JNI code from the implementation.

This introduces Canvas.h which is a pure virtual interface that
is intended to be used by both Skia and HWUI implementation. To help
stage this transition this CL only introduces the interface and Skia
implementation.  The interface is not intended to be final and will
undoubtedly go through iterations in both style and location as we
look to introduce the HWUI implementation.

BUG:15672762
Change-Id: Ibaccdddb87d3b9358f4f0c1d317ead5282d4ee16