Fix alpha when swiping recents on tablets

When you swipe to dismiss a recent item on a
tablet, fade the item to 0 alpha

Also, create common interface for Recents-specific
methods for Recents[Horizontal/Vertical]ScrollView

Bug: 5953654

Change-Id: I0a72b49b3cfae0607b42dbf8f6d4da9898d7e491