Adding a new concept of "next" and "previous" to fragment.s Previously, fragments would
either be placed onto or taken off of the stack, or would just replace the current
fragment. The new next/prev capability gives the ability to run a transition that is
specific to next/previous operations, such as navigating forward and backward in a list.
New next/prev animations may be associated with a fragment replace operation to get the
next/prev animations built into the system (next animates things up, prev animates them
down).
Change-Id: Ia9f3663bac009376420d845b396ac51b8e4d1647
9ff82bf2b3