Ensure that LayerFECompositionState's fence is always non-null

When a solid color layer is overridden by a cached set AND the layer is
skipped due to a prior overridden layer, acquireFence must be nonnull.
Otherwise, the DisplayHardware apis will fail.

To be less error-prone, set LayerFECompositionState's acquire fence to be
Fence::NO_FENCE.

Bug: 209930535
Bug: 208780233
Test: PiP in maps app
Change-Id: Ia896419eb699903453f6f69e4b6c3527e62ec270