[INTERNAL] routing.Router: make getter of its hash changer public
[INTERNAL] routing.Router: make getter of its hash changer public

- The application has the need to set the hash for a router directly. In
  the past, the sap.ui.core.routing.HashChanger.getInstance is used. With
  nested components, the hash changer which belongs to a router should be
  used instead of the global hash changer to keep the hash parts that
  belong to the parent components still in the browser hash. Therefore
  it's required to make this getter public.

Change-Id: Ic835ae1e6f755a9583d3e91e1f6f921b9b18bda6

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5