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.cpp b/libs/renderengine/gl/GLESRenderEngine.cpp
index 3c58238..18eb7b5 100644
--- a/libs/renderengine/gl/GLESRenderEngine.cpp
+++ b/libs/renderengine/gl/GLESRenderEngine.cpp
@@ -515,9 +515,10 @@
     return mDrawingBuffer.get();
 }
 
-void GLESRenderEngine::primeCache() {
+std::future<void> GLESRenderEngine::primeCache() {
     ProgramCache::getInstance().primeCache(mInProtectedContext ? mProtectedEGLContext : mEGLContext,
                                            mUseColorManagement, mPrecacheToneMapperShaderOnly);
+    return {};
 }
 
 base::unique_fd GLESRenderEngine::flush() {