Add new mode for SCHED_FIFO on UI and RenderThreads.

Add a new mode, controlled by sys.use_fifo_ui property, that enables the
top app's UI and RenderThread to be SCHED_FIFO. This eliminates almost
all jank due to scheduling competition with non-UI critical
threads. This mode may not be suitable for all devices.

bug 24503801

Change-Id: I7b8a31830ad80f7efa00236928d5476998ed4e00

libs/hwui/renderthread/RenderProxy.cpp

@@ -514,6 +514,10 @@ void RenderProxy::setProcessStatsBuffer(int fd) {
     post(task);
 }
 
+int RenderProxy::getRenderThreadTid() {
+    return mRenderThread.getTid();
+}
+
 CREATE_BRIDGE3(addRenderNode, CanvasContext* context, RenderNode* node, bool placeFront) {
     args->context->addRenderNode(args->node, args->placeFront);
     return nullptr;