Wire SKSL based stretch shader to HWUI

--Ported SKSL based stretch shader from OpenGL prototype
--Hooked up the stretch APIs in RenderNode to the stretch
shader.
--Updated RenderNode layer logic to promote the RenderNode to
a layer if there is a stretch to be applied to it in order
to feed the layer as input to the stretch shader

Bug: 179047472
Test: builds + sample overscroll stretches + updated CTS test
Change-Id: I744ff70099fe251ce07f23d067bf13444a468c08