When dragging past first or last state, don't reinit target

Example bug:
1. Swipe up to overview and let go
2. Swipe all the way to the top of the screen, past where all apps stops
3. Swipe down

Before this change, you get reset in NORMAL state instead of OVERVIEW.

By ensuring that getTargetState() checks the drag direction before
returning a new state, we guarantee we only re-init in the case that the
state is actually changing. Otherwise it's possible to change the state
to one that is impossible, such as NORMAL when swiping up from ALL APPS.

Change-Id: I19913dded9c94228d06289780b6400e99403f378