[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>
```
```python
with open(manifest_path, 'w') as file_handle:
  xmldoc.writexml(...)
```