Enable acs for hotspot channel selection

Bug: 69065316
Test: tests/wifitests/runtests.sh (on walleye)
Test: manual
Test: wifi sanity (http://b/71636949)

Manual test
-----------
- enable wifi hotspot from settings
$ adb logcat -b main hostapd '*:S' -e 'ACS'
01-04 18:03:53.482  3040  3040 I hostapd : ACS: Automatic channel selection started, this may take a bit
01-04 18:03:53.482  3040  3040 I hostapd : ACS: Offloading to driver
01-04 18:03:53.827  3040  3040 I hostapd : nl80211: ACS Results: PCH: 6 SCH: 0 BW: 20 VHT0: 0 VHT1: 0 HW_MODE: 1
01-04 18:03:53.827  3040  3040 I hostapd : wlan0: ACS-COMPLETED freq=2437 channel=6

Change-Id: I66782e5921db9063acc1dc8433b42edda120278e
6 files changed