Fix flattenLayers_ActiveLayersWithLowFpsAreFlattened

This test crashes on some devices, due to incorrect set up order in the
test. Specifically, the incoming layers were configured as inactive too
early, meaning that setting up the flattener state triggered
RenderEngine expectations that were not yet mocked, which results in
UB.

Bug: 307776985
Test: libcompositionengine_test
Change-Id: I75096c0d043f02f5837a83d0f48053e3cd151691