[Remoting Android] Export CreditsActivity

Currently CreditsActivity is not marked as exported, which means the internal
Help&Feedback library doesn't have permission to launch CreditsActivity and
therefore we can't move the Credits menu item back to Help&Feedback's overflow
menu.

This CL marks CreditsActivity as exported to solve the problem above.

BUG=613604

Review-Url: https://codereview.chromium.org/2384803002
Cr-Commit-Position: refs/heads/master@{#422519}

remoting/android/java/AndroidManifest.xml.jinja2

@@ -51,6 +51,6 @@
             android:uiOptions="splitActionBarWhenNarrow"/>
         <activity
             android:name="org.chromium.chromoting.help.CreditsActivity"
-            android:exported="false"/>
+            android:exported="true"/>
     </application>
 </manifest>