Fix app start logging

Our old trampoline logic doesn't work anymore, as some app starts
are distributed between multiple tasks, which now means multiple
stacks as well.

Instead, track app starts by windowing mode.

Test: Open/close apps, adb logcat -b events
Test: AppStartTests
Change-Id: If106f12ec98fd69ff7d0c4dabff28f88d2096a0b
Fixes: 72953633