Add developer options check for launching Traceur MainActivity

Previously, it was possible for apps to launch the System Tracing
app even when developer options was not enabled.

This change introduces a check at activity start that will close
the activity if developer options are not enabled.

Bug: 142936525
Test: Flashed phone with R master and verified that Traceur could
be launched from the Settings app and via the quick settings tile
long press.
Test: Verified that an app calling startActivity on the Traceur
MainActivity would fail if developer options were disabled.

Change-Id: I8b2da8cf732edf5c44b0a7f85f250752ae7d190f
diff --git a/src/com/android/traceur/MainActivity.java b/src/com/android/traceur/MainActivity.java
index a8c8a2b..be14223 100644
--- a/src/com/android/traceur/MainActivity.java
+++ b/src/com/android/traceur/MainActivity.java
@@ -17,6 +17,7 @@
 
 import android.app.Activity;
 import android.os.Bundle;
+import android.provider.Settings;
 
 public class MainActivity extends Activity {
     @Override
@@ -24,4 +25,16 @@
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity);
     }
+
+    @Override
+    protected void onStart() {
+        super.onStart();
+        boolean developerOptionsIsEnabled =
+            Settings.Global.getInt(getApplicationContext().getContentResolver(),
+                Settings.Global.DEVELOPMENT_SETTINGS_ENABLED, 0) != 0;
+
+        if (!developerOptionsIsEnabled) {
+            finish();
+        }
+    }
 }