Wifi usability: Comments from API review council are addressed

Addressed all comments from API review council:
1. Rename WifiUsabilityStatsListener to OnWifiUsabilityStatsListener.
2. Change the modifier of member variables in WifiUsabilityStatsEntry.java to private and use getters.
3. Document the unit of predicitonHoirzonSec and its expected range.
4. Add @ProbeStaus in probeStatusSinceLastUpdate.
5. Rename probeElapsedTimeMsSinceLastUpdate to
ProbeElapsedTimeSinceLastUpdateMs.
6. Marked @NonNull for Argument 3 in onWifiUsabilityStats.

Bug: 126391553
Bug: 126699865
Bug: 113262380

Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I20d9ad05dc0bb537b09006ca44a447401b3e1001
Signed-off-by: Mingguang Xu <mingguangxu@google.com>