WifiManager: expose Local Only Hotspot APIs

Expose the APIs to start and stop LocalOnlyHotspot.

Bug: 31466854
Bug: 37073685
Test: make update-api
Test: make
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: I8d149ab04d24bf0eb54dd8d8a62e285f9c5b483e