[Android] Merge AndroidManifest.xml from external extension

Add new parameter "manifest" to external extension manifest json file.
This parameter points to an AndroidManifest.xml file, which contains
the config items. These items will be merged to real AndroidManifest.xml,
when packets apk. Please look into the following example.
```
before:
<manifest>
  <application>
    <activity android:name="org.xwalk.app.template.AppTemplateActivity">
    <intent-filter>
    ...
    </intent-filter>
    </activity>
  </application>
</manifest>
```
```
AndroidManifest.xml from exteranl extension:
<manifest>
  <application>
    <activity android:name="org.xwalk.app.template.AppTemplateActivity">
    <meta-data ... />
    </activity>
    <activity android:name="com.google.android.gms.ads.AdActivity" ...  />
  </application>
</manifest>
```
```
after:
<manifest>
  <application>
    <activity android:name="org.xwalk.app.template.AppTemplateActivity">
    <intent-filter>
    ...
    </intent-filter>
    <meta-data ... />
    <activity android:name="com.google.android.gms.ads.AdActivity" ...  />
    </activity>
  </application>
</manifest>
```
Refer to, https://github.com/crosswalk-project/crosswalk-android-extensions/pull/25
In this PR, the AndroidManifest in the extension include other data that we will need in the final manifest.
"is not" -> "does not".
I'd rather mention the name of the file set in the JSON file here, like "Error: %s specified in the extension's JSON could not be found."