Allow for solid color layers to start some candidate cached sets.

This is for letterboxed holepunch: a typical layer stack would be:
1. A solid color letterbox in back
2. A solid color layer for SurfaceView's background protection
3. A game's SurfaceView which has rounded corners

One of the constraints for creating a CachedSet is that it cannot begin
with a solid color layer, under the assumption that if there is an idle
layer on top of a solid color layer, then those two layers should not be
merged into a cached set since displaying the color layer separately is
already cheap. But, for the example layer stack above, this means that
there cannot be a candidate cached set behind the game's SurfaceView,
meaning that there is no hole punch generated, so the game is always in
client composition.

So, let's remove the constraint that a CachedSet cannot start with a solid
color layer for hole punches, which resolves the issue.

Bug: 208780233
Test: Hill Climb Racing with letterboxed rounded corners
Test: libcompositionengine_test
Change-Id: Ia5bcdec363f4401b9b9738c5f6ce335497d6362b