SurfaceFlinger: Add childLayersOnly variant of capture layers.

Currently captureLayers is being used to capture children of abstract
buffer-less layers, for example we capture the node representing
a Task. Because of animations, etc, the node may have been scaled
when we want to take the screenshot, so we solved this problem with
an inverse transform in LayerRenderArea. However we have parallel problems
for crop, visibility, etc...It seems what we really want from captureLayers
is the ability to capture all child layers while ignoring the parent imposed
state as if in a parallel hierarchy. I initially considered implementing a parallel
hierarchy with layer clones but it seemed much less risky for P to instead
reparent the existing hierarchy to a parallel abstract parent on the server side.

Bug: 72760590
Test: Existing tests pass. Quickstep works with new implementation. New transaction tests.
Change-Id: Ifd2b2f9ed45351d8ed0d7f09be1cd9806ec4abe8