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/gl/GLESRenderEngine.h b/libs/renderengine/gl/GLESRenderEngine.h
index e7ed9c0..1ff3674 100644
--- a/libs/renderengine/gl/GLESRenderEngine.h
+++ b/libs/renderengine/gl/GLESRenderEngine.h
@@ -57,7 +57,7 @@
                      EGLSurface protectedStub);
     ~GLESRenderEngine() override EXCLUDES(mRenderingMutex);
 
-    void primeCache() override;
+    std::future<void> primeCache() override;
     void genTextures(size_t count, uint32_t* names) override;
     void deleteTextures(size_t count, uint32_t const* names) override;
     bool isProtected() const override { return mInProtectedContext; }