[Android] Implement the feature of enabling immersive mode for full screen.

Since Android 4.4 (API Level 19) introduces the immersive related mode
for full screen, in Crosswalk, we need to support this feature accordingly.
This PR is resolving this issue by implementing the specific methods
corresponding to the scenarios that full screen mode is turned on and off.

BUG=https://crosswalk-project.org/jira/browse/XWALK-979
Merge pull request #1626 from ShDavidliu/Immersive

[Android] Implement the feature of enabling immersive mode for full scre...
[Runtime] Separate WGT application manifest handling from XPK applications.

When loading WGT manifest file, current Crosswalk maps XML Widget
elements to manifest keys under namespace 'application_manifest_keys' by
hard code. Consequently, WGT app and XPK app share the same manifest
keys and manifest handlers. As with more manifest keys and handlers are
introduced for WGT app, this causes a mix up of WGT specific keys and
XPK specific keys, due to WGT and XPK app have different manifest formats
and keys, especially put Tizen Extending Configuration Elements into
consideration.

A solution to fix above issue is to separate WGT manifest keys and handlers
from XPK apps. In detail, when loading WGT manifest, we introduce a
mechanism to save XML configuration information into manifest Dictionary
structure(note that this is not hard code mapping), keys are different
from those of XPK apps. WGT keys are put under 'application_widget_keys'
namespace, while XPK keys are put under 'application_manifest_keys'.
Then we could add WGT specific handlers for WGT keys.