Adding new APIs to save state and retrieve it

The TaskFragmentOrganizer now can save state in the system, which
could be retrieved later if the TaskFragmentOrganizer was killed
by the system (or app just randomly crashed) and re-registered again
once the process restarted.

Bug: 289875940
Test: wm presubmit
Flag: com.android.window.flags.ae_back_stack_restore
Change-Id: I14942b70121133c2031acfa143eddfc51e9bffa5