Fix crash when Paths are GCd in hw accelerated apps

A recent change to optimize path rendering didn't account for
the destruction of native objects by the VM finalizer. We may be
done with the Java level version before we're done with the native
structure that's used by the display list. For example, a drawing
method on a View that creates a temporary path to render into the
canvas will implicitly create a native structure that is put onto
the GL display list. That temporary path may go away, but the native
version should stick around as long as the display list does.

The fix is to refcount the original native version of the path
and only delete it when the refcoutn reaches zero (which means that
it is no longer needed by any display list). This is a similar mechanism
used for bitmaps and shaders.

Change-Id: I4de1047415066d425d1c689aa60827f97729b470