[RCD] Check the RCD layer on the top

RCD layer can be only placed on the top of the layer stack. To maintain
the correctness of blending result, we make the RCD layers fallback to
client composition when they are not on the top layer.

Bug: 233282377
Test: Enable RCD layer on the top
Change-Id: I8f241dfb61a31ef607c979087f8c616ceb889abb