ScanOnlyModeManager: add NetworkObserver

Add NetworkObserver logic to ScanOnlyMode to detect when the underlying
interface is up/down.

Bug: 32659843
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Change-Id: If3bd7378816c7b8cfdc6de9639cd4b9eea80d91b
5 files changed