Exclude TaskSnapshotManager from ActivityThread

This is a temporary solution to fix the increase observed in the
RSS "shared clean" memory section in every forked process.

Bug: 436397802
Flag: com.android.window.flags.reduce_task_snapshot_memory_usage
Test: atest system-processes-memory
Change-Id: Id2a5e5744e36b454940a414933607227e5343ed5