[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
﻿[Android] Enable the io thread client in xwalk content.

  The io thread client is implemented while not enabled in xwalk content,
this fix is to enable the io thread client in xwalk content and adding the
related handler into the xwalk content native layer and java layer.

BUG=https://crosswalk-project.org/jira/browse/XWALK-653
Merge pull request #1255 from xingnan/app-shcheme

App scheme
Merge pull request #1309 from ShDavidliu/io_thread_client

[Android] Enable the io thread client in xwalk content.
[Runtime] Polish application browser test

1. Add a new xwalk.app.test.notifyTimeout API to notify test timeout.

2. Make test API extension running on UI thread. And let the client to return
the xwalk.app.test.notifyPass/Fail/Timeout callback explicitly when it's ready.

3. Using Promise to provide a common asynchronous test driver. The JS can now
write code like below to test:
  function test1(resolve) { ... } // calls resolve() when test is done
  function test2(resolve) { ... } // calls resolve() when test is done
  var tests = [test1, test2];
  xwalk.app.test.runTests(tests, 5000); // if tests can't finish in 5 seconds
  then a TIMEOUT notification will be sent.