Unregister task fragment organizer when dream is destroyed.

Fixes: 350734809
Test: android studio profiler with heap dumps
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:11169ab775550ac71f2e020bd5600b1bdd9118db)
Merged-In: I34e85d9decc13c173840137cd3783215c64203d4
Change-Id: I34e85d9decc13c173840137cd3783215c64203d4