MacAddress: add @NonNull annotations

This patch adds to the MacAddress class:
  - getAddressType() method which replaces addressType(), for naming
  consistency
  - @NonNull annotations on all input and output reference values for
  all public methods (@hide and public).

TYPE_UNKNOWN, which currently cannot be observed with a non @hide
method, is also removed from the public api.

Bug: 71866627
Test: $ runtest frameworks-net
Change-Id: I2af70408d46f431b7b32183e6b48ddae9a261a2c