autotest: Modify hostap_config to support late frequency modification

In situations where we want to run the same configuration but on several
different channels/frequencies, it is convenient to support writing:

ap_config.channel = 11

Rework the channel and frequency properties to properly support this
kind of behavior.

BUG=chromium:271515
TEST=A pending test that consumes this logic works just fine.  We can
observe the different hostapd config files being written out correctly
in logs.

Change-Id: Iddd5518041f3c19806e546afa4d224ef05dd1055
Reviewed-on: https://gerrit.chromium.org/gerrit/65643
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
1 file changed