Fix NPE and make getTask return Optional

Fix an NPE when attempting to update the thumbnail in IconRecentsView.
If the task item is binding the loading UI, it has no task so the app
will crash, so we just need to ensure the task is actually present
first.

While we're here, also change the API to return an Optional<Task> and
make a tighter contract, making it harder to make this mistake in the
future.

Bug: 130746661
Fix: 130746661
Test: Go from app => overview on recents Go
Change-Id: I1402fcd2e58bdeb703c7dcc1b9dcf0d258808b3d
(cherry picked from commit 77f01524)