WifiManager: implement watch LocalOnlyHotspot

Implement new calls to watchLocalOnlyHotspot and
unregisterLocalOnlyHotspotObserver along with the classes
LocalOnlyHotspotObserver and LocalOnlyHotspotSubscription.

Added tests for watching LOHS and cancelling a LOHS subscription.

The calls will be exposed in a later CL.

Bug: 36704763
Test: compiles
Test: frameworks/base/wifi/tests/runtests.sh
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Change-Id: Ia0a528191ae0897742304d8b61e9779ad721a450