Add Systrace support for API 18+ in OSS

Differential Revision: D2627757

fb-gh-sync-id: a01347800d8e9ffda8759cc17df04f7cd139b17c

Author: astreet

ReactAndroid/src/main/java/com/facebook/systrace/Systrace.java

@@ -9,8 +9,12 @@
 
 package com.facebook.systrace;
 
+import android.os.Build;
+import android.os.Trace;
+
 /**
- * Systrace stub.
+ * Systrace stub that mostly does nothing but delegates to Trace for beginning/ending sections.
+ * The internal version of this file has not been opensourced yet.
  */
 public class Systrace {
 
@@ -50,9 +54,15 @@ public static void traceInstant(
   }
 
   public static void beginSection(long tag, final String sectionName) {
+    if (Build.VERSION.SDK_INT >= 18) {
+      Trace.beginSection(sectionName);
+    }
   }
 
   public static void endSection(long tag) {
+    if (Build.VERSION.SDK_INT >= 18) {
+      Trace.endSection();
+    }
   }
 
   public static void beginAsyncSection(