Adding BackupHelper to help backup and restore AE state

The backup will be scheduled whenever a TaskFragmentContainer is
updated and the backup is done while the app is idled.

Bug: 289875940
Test: wm presubmit
Flag: com.android.window.flags.ae_back_stack_restore

Change-Id: Ie9d6d7f235269b9e4b321d51beea28b3e84274f7