Update Overview content descriptions.

- If there is a task label with a provided task description, ensure that
  it is used instead of the activity label when generating the content
  description (the concatenation of the badged application name and
  activity label.
- Also fix a case where content descriptions can go stale if they are
  updated between first preloaded. Just don't cache the content
  description if there is a task description.

Bug: 37417229
Test: Launch Play store, go to Overview
Change-Id: Ieeef9ecd93759c130dc66902f249783e7d218be7