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/SkiaGLRenderEngine.h b/libs/renderengine/skia/SkiaGLRenderEngine.h
index 97d3b72..145f2dd 100644
--- a/libs/renderengine/skia/SkiaGLRenderEngine.h
+++ b/libs/renderengine/skia/SkiaGLRenderEngine.h
@@ -53,7 +53,7 @@
                        EGLSurface protectedPlaceholder);
     ~SkiaGLRenderEngine() override EXCLUDES(mRenderingMutex);
 
-    void primeCache() override;
+    std::future<void> primeCache() override;
     status_t drawLayers(const DisplaySettings& display,
                         const std::vector<const LayerSettings*>& layers,
                         const std::shared_ptr<ExternalTexture>& buffer,