- Always return a copy of the task list to ensure that the model doesn't
hold refs to the thumbnail data if it was loaded into the same task
- Always clear the task thumbnail data ref once the visibility of the
task view changes to be invisible
Bug: 132309376
Test: Enter overview scroll to the end of the list, relaunch app and take
heap dump
Change-Id: I4437fd30172a5fe2a78c111f780163a1e6bbbb54
Winson Chung
df9fc63e8c