WifiMetrics: add time since boot to ConnectionEvent

ConnectionEvent only records Unix time, while
StaEvent records time since boot. This makes it
difficult to join the two lists.

Thus, add time since boot to ConnectionEvent too,
and deprecate the Unix timestamp.

Remove mRealStartTime since that stores the same
information, but it is only stored in memory and
not uploaded.

Bug: 176929711
Test: atest FrameworksWifiTests
Updated-PDD: TRUE
Change-Id: I5663f0d92b641c3224c873bde50bab1a8f533ccd
2 files changed