CaptureLayers: Avoid promoting parent on binder thread

We promote the parent of the root layer when we call getLayerStack.
Since we don't hold an IBinder to the root layers parent, just to
the layer itself, this could create a situation where we are the last
reference to the layer. Layers have to be destroyed on the main
thread and so that would be invalid. Hopefully this is the last
case and now we can start getting rid of refbase for layer.

Bug: 220176775
Bug: 223069308
Bug: 223081111
Test: Existing tests pass
Change-Id: I37a0834ddac6d8e84170674aba0c49268d65fa11