[Tizen] Add overlay mode to TopViewLayout

In overlay mode the top_view_ will be over the content_view_.
[Tizen] Add WatcherClient interface and use it on the indicator
[Tizen] Add WidgetHolder and TizenSystemIndicatorWidget

The system indicator is implemented as a widget because it needs to
receive events and may also be an overlay on top of the rest of the
content, regular views do not support this. We add it to the holder,
that is a view that doesn't draw anything, just passes the bounds
information to the topview layout.