Allow overriding ResourceManager in ApplicationContext

For performance reasons, we need to skip the registering of resources in
the test suite.

Allow to subclass ApplicationContextImpl and to override the creation
of the ResourceManager. This results in a cleaner design than patching
the ApplicationContext at runtime.

Change-Id: Ie338dcb66103f49ae22aa2ff23266f260937ae1e
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>