Logs the activated session duration for magnification. (1/2)

Uses the atom MagnificationTripleTapAndHoldActivatedSessionReported in
westworld to log the activated session duration for the window magnifier
which is triggered by the triple-tap-and-hold gesture.

The duration is from the window magnifier's enabled time to its disabled
time.

Bug: 216733714
Test: atest WindowMagnificationGestureHandlerTest
      m statsd_testdrive & statsd_testdrive 452
Change-Id: I423ac39c167c89b0b2db931a616cdd34d6eb151c