startop: Add timestamps for IntentStarted and ActivityLaunchFinished.

The timestamps are used to compute duration of TotalTime and ReportFullyDrawn.

TotalTime = t(ActivityLaunchFinished) - t(IntentStarted)
ReportFullyDrawn = t(ReportFullyDrawn) - t(IntentStarted)

where t(x) is the timestamp of x.

Bug: 137786390
Bug: 137784595
Test: Make
Test: atest CtsWindowManagerDeviceTestCases:ActivityMetricsLoggerTests
Change-Id: I75c0be8508499dce186fd392e79b67b2fefc8842