Refer to outer components in TaskFragmentOrganizer via WeakReference.

TaskFragmentOrganizer is handed over a binder when registered. As a
result, any references to outer components should be done via a
WeakReference. This changelist makes these adjustments.

Test: manual. Monitored heap usage
Flag: EXEMPT bugfix
Fixes: 360088321
Change-Id: Ia920df9359105ad928c45cbdeaaa4edbe9af3b2e