Wifi usability: Add timestamp in WifiUsabilityStats

Together with the timestamps in each entry (WifiUsabilityStatsEntry) of WifiUsabilityStats, the added timestamp allows us to check whether the WifiUsabilityStats is obsolete or not.

Bug: 129282057
Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I1c968786173f0e719b10651315a6d1bb5b42438f
Signed-off-by: Mingguang Xu <mingguangxu@google.com>