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/libs/renderengine/threaded/RenderEngineThreaded.h b/libs/renderengine/threaded/RenderEngineThreaded.h
index eb6098e..76b9016 100644
--- a/libs/renderengine/threaded/RenderEngineThreaded.h
+++ b/libs/renderengine/threaded/RenderEngineThreaded.h
@@ -42,7 +42,7 @@
 
     RenderEngineThreaded(CreateInstanceFactory factory, RenderEngineType type);
     ~RenderEngineThreaded() override;
-    void primeCache() override;
+    std::future<void> primeCache() override;
 
     void dump(std::string& result) override;
 
@@ -74,6 +74,7 @@
 private:
     void threadMain(CreateInstanceFactory factory);
     void waitUntilInitialized() const;
+    static status_t setSchedFifo(bool enabled);
 
     /* ------------------------------------------------------------------------
      * Threading