MacAddress: light refactors and improve documentation

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