Fix incorrect calculation of activity snapshot ID

When a task change is involved in an activity transition, the activity
snapshot ID can be incorrectly calculated as zero. This occurs if a
duplicate activity is added to ActivitiesByTask.
In addition to the incorrect ID, the snapshot is also cleared
immediately because putSnapshot is called again with the same activity.

Flag: EXEMPT bugfix
Bug: 424273966
Test: atest ActivitySnapshotControllerTests
Change-Id: I9c4d4a46fb60706363da8ddb851b0fd930168193