Place mirrored layers offscreen instead of at root.

Mirrored Layers should never be placed at the root of the layer
hierarchy. Instead, they should be created offscreen and allow the
caller to place the layer where they want in the hierarchy. This
behavior was changed with the introduction of LayerCreatedState.

Send placeAtRoot flag when creating a layer to allow different create
callpoints to specify whether they want the layer created to be placed
at the root.

Test: Window Magnification no longer flickers.
Test: MirrorLayerTest
Fixes: 192536474
Change-Id: I08f47b1b1f19b7c655c3687cb4c1fa64844e2cc5