Return const ref to RouteDistinguisher

Automatic copy constructor and assignment operator do not work for
RouteDistinguisher.
Return const refs to this class instead of returning by value.
The only exception is the static ::FromString method.

Change-Id: I68d4d8694966f1ad9d485c1cf34bc5a19229815c
Partial-Bug: 1399603
Merge "Return const ref to RouteDistinguisher" into R1.10