Use HostapdManager from libwifi-system

This class wraps the logic being removed from this file.  This will
make moving equivalent logic to wificond easier.

Bug: 30040724
Test: netd continues to be able to set up APs for tethering using
      libwifi-system rather than previous code.
Test: No SELinux policy violations during/after setup.
Test: Can sign in to network backed by hostapd started by netd using
      libwifi-system.

Change-Id: I27e2e53594ec2ae4ef4150c63c8604bb34700af0
(cherry picked from commit 0a9eb47d)