[Extensions] Add support for persistent callback listener

This is going to be the base of the EventTarget implementation. It is
important to remove the listener at some point otherwise the object will
leak.
[Extensions] Add browsertests for the persistent callback listener

The test implements a heartbeat which works in a similar way of an EventTarget.
The callback is set only one and gets called multiple times.
[Extensions] Remove development debug statements

These debugging statements were useful during development, but now
as we are feeling confident that everything should be working, they
can be removed.
[Android] Disable the unit test for DeviceCapabilities.

It's failed in both shared and embedded modes. Need to fix them.
Create one issue to track it:
https://crosswalk-project.org/jira/browse/XWALK-102
Merge pull request #1045 from yongsheng/master

[Android] Disable the unit test for DeviceCapabilities.
Merge pull request #1039 from vcgomes/remove-development-debugs

[Extensions] Remove development debug statements
Merge pull request #1038 from tmpsantos/persistent_callback

[Extensions] Support for persistent callback
[Android] Use loadAppFromManifest if a manifest.json is used for a web app.

Add loadAppFromManifest support for web app. If web app is packaged with
manifest, directly use loadAppFromManifest API to load app.
Because native manifest parser only supports original proposal
"app:launch:local_path", manifest related test cases are modified.
When the specfication for manifest is stable, will revisit these test
cases and native manifest parser.

BUG=https://crosswalk-project.org/jira/browse/XWALK-100?filter=10007