Persist task snapshots to disk

So they can be used again after rebooting or when the process gets
killed, but the snapshot is still used for recents.

Also implement TaskSnapshotLoader, to restore it from disk. The
infrastructure around restoring and caching snapshots for recents
will be implemented in the next CL.

Test: runtest frameworks-services -c
com.android.server.wm.TaskSnapshotPersisterLoaderTest

Bug: 31339431
Change-Id: Iaec03c4cc92e04b6dd7e623bca755ddc92613bce