Move fully visible task logic to more centralised method.

This means that overlay is initialised in all cases, not just some. Also uses pre-existing method for `isTaskViewFullyVisible()`

Fix: 376264918
Test: Manual checking of flows in bug.
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Change-Id: Ia898a2a869b79fb120a162fa5a3fe7df0fbf9800