[Android] Put app resource in sub directory of assets

Before this all app resources are put in assets, which may cause
confliction with other resources(of runtime, extensions etc).
So we define a sub directory "www" to put the app resources
instead.

BUG=https://crosswalk-project.org/jira/browse/XWALK-174
[Android] Add app URI scheme support

1 App URI scheme is part of sysapps API, http://app-uri.sysapps.org
2 On Android the local resources are located in assets, which could
be accessed by file:///android_asset/ scheme. So we bridge the app
URI to file URI.
3 Add the HTTP response support(200, 400, 403, 404, 501).

BUG=https://crosswalk-project.org/jira/browse/XWALK-174