[Runtime] Fix RuntimeContext ownership and unify its creation on all platforms

This patch makes RuntimeContext owned by XWalkContentBrowserClient which is better than previous solutions:
- On Android RuntimeContext used to be a singleton which is wrong as XWalkContentBrowserClient (which creates a RuntimeContext instance) is already a singleton.
- On other platforms RuntimeContext used to be owned by XWalkBrowserMainParts which is wrong too as XWalkBrowserMainParts is a strategy class and it is not supposed to own global objects.

Also this patch unifies the creation of RuntimeContext between android and others removing ugly '#ifdefs' and unneeded difference in the code path.