Fix NullPointerException in ListView

There is a NullPointerException in `handleHorizontalFocusWithinListItem()`
because `selectedView.findFocus()` returns null and then there is no null
check when when the assigned variable `currentFocus` is used, although
`View.findFocus()` states that it may return null.

Change-Id: I6897027e9a2a238d9283e6b9f5146198989fcac0