Tethering: interface updates can be handled again

When reloading wifi firmware, unsolicited responses from netd were
processed after softap had started and caused wifi tethering to be torn
down.

The NetworkManagementServer.wifiFirmwareReload call has been changed to
not only block for the command to finish, but also until all unsolicited
messages (interface updates) have been handled.  We should now be able
to handle interface updates in tethering without suffering from the
softap bringup/interface down notification race condition.

BUG: 27857665
Change-Id: Ie57cb8f760781b3227df575b577b33667070d63e