Implement swipe to delete on Recents Go

Introduce a ItemTouchHelper.Callback implementation that hooks up remove
logic to swiping right on task items. Additionally, tighten the
modification access to the task list in TaskListLoader so it is harder
to mis-use.

Bug: 114136250
Test: Removed app via swiping
Change-Id: I9b6e2783d1f6d56e655e9b7ffa9af85328f98159