Support to record multiple activities in the same task as one snapshot.

So the snapshot can be easier to be used with current SnapshotDrawer,
also the overall structure would be much similar with associated task
starting window.

- Record a single activity snapshot while multiple activities in the
  same leaf task are closing together, this can be extention if mulitple
  adjacent will happen.
- Share the snapshot from activities: because the key of cache must be
  an activity, so each activity can be the key to find this snapshot
  from cache, but the activities must be exactly match when calling
  getSnapshot.
- Remove the snapshot if one of activity removed, usually the
  configuration of rest activities will be changed, so doesn't need to
  keep the snapshot.

Bug: 259497289
Bug: 274997067
Test: atest ActivitySnapshotControllerTests
Test: test on sample app, monitor the snapshot of adjacent activities
should work like single activity snapshot.

Change-Id: I357010ab79eda165564e36fe7203fc508dbc357b