Report launch time on the last started activity

Assume:
L is "Activity launched".
D is "Window drawn/displayed".
T is a trampoline activity (e.g. a splash screen).
M is the main activity.

An application may be designed as:
L(T) -> L(M) -> D(T) -> D(M)

Originally we will report the launch time on D(T).
Now if L(M) happens before D(T), then it is supposed
to be a continuous launch sequence and report the
launch time on D(M).

Bug: 132736359
Test: atest ActivityMetricsLoggerTests
      atest ActivityMetricsLaunchObserverTests# \
            testOnActivityLaunchFinishedTrampoline

Change-Id: I9ffd3b94d76831572ea961a8dcbf9d760a617430