Add a BackoffSequence utility; use it for Private DNS validation

Add a simple, if verbose, BackoffSequence class to encapsulate some
RFC 3315 section 14 style mechanics.

Test: as follows
    - built
    - flashed
    - booted
    - system/netd/tests/runtests.sh pass
    - make netdutils_test && \
      adb push .../data/nativetest64/netdutils_test/netdutils_test /data/nativetest64/netdutils_test && \
      adb shell /data/nativetest64/netdutils_test passes
Bug: 64133961
Bug: 72344805
Change-Id: Ib15a9454e17529a735bca4d9a0e96de8baae84c3