[Android] Add getDisplay() to XWalkDisplayManager

WiDi PR #965 add XWalkDisplayManager class referenced from
DisplayManager class introduced since Android 4.2 (API Level 17).

While getDisplay() is not included because WiDi does not use it.
This PR is adding that this api and implementation.
[Android][Extensions] Implemented W3C SysApps Device Capabilities API

The implementation is based on 17 October 2013 editor draft version.
CPU, Memory, Storage and Display are included, the Audio and Video Codecs
is not implemented yet.

SPEC=http://www.w3.org/2012/sysapps/device-capabilities/
BUG=https://crosswalk-project.org/jira/browse/XWALK-48
Merge pull request #961 from halton/device_capabilities

[Android][Extensions] Implemented W3C SysApps Device Capabilities API