[SysApps] Introduce the SysAppsManager

This class will be the single entry point for registering all
the SysApps extensions. It should be the only class of the "sysapps"
namespace exposed to other subsystems.

In the future it will also manage the lifecycle of data providers (platform
abstractions for extensions like DeviceCapabilities).
[SysApps] Add a "sysapps" master flag

A single flag to disable all the SysApps APIs.
[SysApps] Add unit tests for the SysAppsManager
Merge pull request #1357 from tmpsantos/sysapps_manager

[SysApps] Introduce the SysappsManager
[Runtime] Move application runtime& event extension under xwalk::application namespace.

Present ApplicationEventExtension & ApplicationRuntimeExtension are
located inside namespace "xwalk". Actually they are parts of
"application module". This PR moves the two inside namespace
"xwalk::application".