Remove unnecessary drawing commands when drawing shadows.

When blurring behind a shadow, this will ensure that it
is only blurred once. It does so by updating
Layer::prepareShadowClientComposition to add ShadowSettings
to the input layer, rather than adding an extra layer.

Collapsing the shadow layer into the same layer as the caster will
result in undefined behavior in the old GLESRenderEngine.

Bug: 188050128
Bug: 175915334
Test: atest SurfaceFlinger_test and atest librenderengine_test
Change-Id: I5b74ef9659b519fcc6b89dfd873c35fc579fddd7