Make layer double drawing visible in overdraw debug mode

A clipped saveLayer will cause two draws - first to an onscreen
buffer, then again back to the framebuffer. This change - when in
overdraw debug - reissues draws associated with a clipped saveLayer,
but only to the stencil buffer. Operations within a saveLayer are
shown correctly to be twice drawn, and View.setAlpha() without an
associated hardware layer, or hasOverlappingRendering() are made more
visible.

Hardware layers, on any frame that they are updated, similarly draw
twice, and will also be counted against the stencil buffer doubly.

Note: greater depths of layers - e.g. a saveLayer within a saveLayer -
are not respected, as that would require additional region tracking.

Change-Id: I61fb0e61038fe66013d59914c20bb47a550dea7d