Some app launches go through different stacks. However, in P we
have one stack per task, which broke launch time tracking for
certain apps.
We fix this by tracking launch time per windowingMode, like we
do in ActivityMetricsLogger.
Test: Delete Chrome app data (to ensure FirstRunActivity gets
invoked, am start -n com.android.chrome/com.google.android.apps.chrome.Main -W
Fixes: 78302265
Change-Id: I739e63aeaf04d6336621e3d61f065259e518d9b4
de8305a566