View "lifecycle" not handled correctly w.r.t handler that refreshes display text.

The method `startTaskForPeriodicallyUpdatingRelativeTime()` is called both in `onAttachedToWindow()` and in `onVisibilityChanged()`. This could result in the `Runnable` being scheduled twice.

This is because the first time the `RelativeTimeTextView` is added to its parent, both `onAttachedToWindow()` and `onVisibilityChanged()` are called (the latter is called for the parent Window).
