Use ImageReader to persist task snapshot to disk.

Which can prevent gl_texture being created for SkImage while copy a
hardware bitmap to software bitmap. So the system server won't has to
ask for an extra dmabuf during persist task snapshot.

Flag: com.android.window.flags.reduce_task_snapshot_memory_usage
Bug: 238206323
Test: dump dmabuf everytime after a close task transition finish,
verify the dmabuf won't increase in system server. Also since the task
snapshot will be removed after open transition, verify the snapshot in
the dmabuf will be removed.

Change-Id: I03f25c097bfdda5cc9ca9b6c33a30fc980103c0b