Don't export LauncherShortcutActivity

This activity was inadvertently changed to exported in
https://codereview.chromium.org/2268323003/

BUG=645310

Review-Url: https://codereview.chromium.org/2328843002
Cr-Commit-Position: refs/heads/master@{#417600}

chrome/android/java/AndroidManifest.xml

@@ -254,7 +254,7 @@ by a child template that "extends" this file.
             android:theme="@android:style/Theme.NoDisplay"
             android:taskAffinity=""
             android:excludeFromRecents="true"
-            android:exported="true" />
+            android:exported="false" />
 
         <!-- Upgrade related -->
         <activity android:name="org.chromium.chrome.browser.upgrade.UpgradeActivity"