Distinguish warm/hot launch by whether the activity has attached process

If the process of an activity was died in background, its activity and
task record will still be in history stack. If the process is started
again in background such as receiving broadcast. Then the launch of the
same activity will have the conditions: process exists and the result
code is START_TASK_TO_FRONT. But the activity still need to create so
it should be warm launch.

Fixes: 171549711
Test: ActivityMetricsLaunchObserverTests#testOnActivityLaunchFinished
Change-Id: Iaf53ab8a35192249c20a73d9cb0de2a5e8effc64