[Android] Enable instrument test for Messaging API.

Enable instrument test for Messaging API.
[Android] Support 'xwalk_description' field in manifest.

Support 'xwalk_description' field in manifest according to W3C manifest spec.
The legacy field 'description' are still supported, but will print a warning message.
Merge pull request #2226 from jinbang/tool_support_manifest_xwalk_description

[Android] Support 'xwalk_description' field in manifest.
Merge pull request #2239 from shawngao5/enable_messaging_test

[Android] Enable instrument test for Messaging API.
[Android] Rename version to xwalk_version in manifest and make it optional

According W3C manifest spec, 'version' field of manifest should be:
1) renamed to "xwalk_version"
2) optional
3) the "version" field should be handled if present, but issue a deprecation warning

BUG=https://crosswalk-project.org/jira/browse/XWALK-2053