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/skia/SkiaRenderEngine.h b/libs/renderengine/skia/SkiaRenderEngine.h
index 308c5ff..4cda133 100644
--- a/libs/renderengine/skia/SkiaRenderEngine.h
+++ b/libs/renderengine/skia/SkiaRenderEngine.h
@@ -39,7 +39,7 @@
     SkiaRenderEngine(RenderEngineType type) : RenderEngine(type) {}
     ~SkiaRenderEngine() override {}
 
-    virtual void primeCache() override{};
+    virtual std::future<void> primeCache() override { return {}; };
     virtual void genTextures(size_t /*count*/, uint32_t* /*names*/) override{};
     virtual void deleteTextures(size_t /*count*/, uint32_t const* /*names*/) override{};
     virtual bool isProtected() const override { return false; } // mInProtectedContext; }