WifiManager: expose base LOHS apis

This CL exposes a minimal set of APIs to start LocalOnlyHotspot.  This
includes the call to start the hotspot along with the objects necessary
to get updates and unregister the hotspot request.

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: I5e9a6a842c0b9233ebf99317f61e0b36192d5be7