[Android] Support permissions for Java external extensions

Following changes are made:
1. merged external extensions permissions to Android manifest file
2. added a unit test about this feature
3. fixed empty extension_name issue caused by redundant separators

BUG=https://crosswalk-project.org/jira/browse/XWALK-160
Merge pull request #1092 from DonnaWuDongxia/permission

[Android] Support permissions for Java external extensions