Bump refcount of canvas created for Picture

Both the Canvas and Picture objects will unref the canvas in their
finalizers, so an extra ref is needed. This had been present but was
inadvertently removed in the patch "Refactor android.graphics.Picture
JNI bindings." This patch simply restores the previous ref and comment.

Bug: 16399257
Change-Id: I9911826bd0e668bd25546ef54b0c9c1d467538c8