SF: RenderEngine::primeCache should not be called as SCHED_FIFO

RenderEngine::primeCache is quite long and as such can't run at
SCHED_FIFO.

Test: boot the device and observe RenderEngine::primeCache timing
Bug: 190529454
Change-Id: I9024c4a365efde717bd4cc9f382d1729a2e2446f
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 519a5ab..a3fa8d6 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -1183,6 +1183,8 @@
     sp<StartPropertySetThread> mStartPropertySetThread;
     surfaceflinger::Factory& mFactory;
 
+    std::future<void> mRenderEnginePrimeCacheFuture;
+
     // access must be protected by mStateLock
     mutable Mutex mStateLock;
     State mCurrentState{LayerVector::StateSet::Current};