Add a marker to indicate when JS thread priority is lowered

Summary:
When RN starts up, it lowers the default priority of the JS thread. This diff sets a point to see when the JS thread priority is lowered.

In subsequent diffs, we will be able to use this marker to play around with bumping the priority of the JS thread till TTI is done.

Reviewed By: alexeylang

Differential Revision: D8965457

fbshipit-source-id: 87cb1e3d3b370af183f388c411fd9a87a6cba250

ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java

@@ -25,6 +25,7 @@
 import static com.facebook.react.bridge.ReactMarkerConstants.REACT_CONTEXT_THREAD_START;
 import static com.facebook.react.bridge.ReactMarkerConstants.SETUP_REACT_CONTEXT_END;
 import static com.facebook.react.bridge.ReactMarkerConstants.SETUP_REACT_CONTEXT_START;
+import static com.facebook.react.bridge.ReactMarkerConstants.CHANGE_THREAD_PRIORITY;
 import static com.facebook.react.bridge.ReactMarkerConstants.VM_INIT;
 import static com.facebook.react.uimanager.common.UIManagerType.FABRIC;
 import static com.facebook.systrace.Systrace.TRACE_TAG_REACT_APPS;
@@ -1024,6 +1025,7 @@ public void run() {
           @Override
           public void run() {
             Process.setThreadPriority(Process.THREAD_PRIORITY_DEFAULT);
+            ReactMarker.logMarker(CHANGE_THREAD_PRIORITY, "js_default");
           }
         });
     reactContext.runOnNativeModulesQueueThread(

ReactAndroid/src/main/java/com/facebook/react/bridge/ReactMarkerConstants.java

@@ -25,6 +25,7 @@ public enum ReactMarkerConstants {
   NATIVE_MODULE_INITIALIZE_END,
   SETUP_REACT_CONTEXT_START,
   SETUP_REACT_CONTEXT_END,
+  CHANGE_THREAD_PRIORITY,
   CREATE_UI_MANAGER_MODULE_START,
   CREATE_UI_MANAGER_MODULE_END,
   CREATE_VIEW_MANAGERS_START,