Backport Pedro's fix for bug 1399603

Automatic copy constructor for MacAddress doesn't work as expected.
Add DISALLOW_COPY_AND_ASSIGN to prevent the copy constructor from being
used.

Change-Id: Id8ad9456f64a574c2bade9ed584614260cb9b8f6
Partial-Bug: #1399603
Merge "Backport Pedro's fix for bug 1399603" into R2.0