Move AP Band Conversion notification to Settings

SettingsBackupAgent will send out the notification
itself instead of delegating it to WifiManager.

Bug: 144218444
Test: Removed AP Band conversion check to always trigger
notification when restoring from backup.
Follow steps at: https://developer.android.com/guide/topics/data/testingbackup
Verified that notification is displayed when restoring
from backup.
Verified that expanding notification by dragging down
shows full notification text.
Verified tapping on the notification opens the tethering
Settings page, and also dismissed the notification.

Change-Id: I729b1cbf443229687c086982d51b96f326b534e1