Safeguard against uncommon scenarios when pruning

E.g. more than one session by activity token

Also
- use cheaper datastructure to store sessions temporarily.
- Fix dumping when there are not responses

Fixes: 38203308
Test: Created two sessions, put them in background. Killed the apps, killed the tasks.
      Waited 30 second and started new sessions -> Saw sessions pruned.
Change-Id: I8fe94773a3381617779e5bdded317f3744c831e5