[Runtime] Move "is_running_as_service" flag to XWalkRunner

This distinction will be used later in xwalk_paths, to setup different
data paths for Crosswalk in Tizen.
Merge pull request #1408 from cmarcelo/run-as-service-switch

[Runtime] Move "is_running_as_service" flag to XWalkRunner
Prepare ApplicationService for multiple applications handling.

Prepare ApplicationService for multiple applications handling: ApplicationService now owns a ScopedVector of Applications.
Application::Observer class is introduced so that application can notify about its termination and deletion.
Application::Launch methods return a pointer to the corresponding Application instance, ApplicationSystem methods are modified accordingly.