This patch does some light refactoring in MacAddress to prepare for
exposing MacAddress in the public api:
- documention is improved
- some method names are renamed
- a toSafeString method is added
- a padding bug in the conversion methods outputting strings for
mac addresses is fixed
Bug: 69390696
Test: runtest frameworks-net
Change-Id: I399a97dabc2dfa8df9c5518c8b12484e43ca05c9