Don't allow swiping to HintState if we're already in HintState

Context: there was a bug where you could get stuck in HintState if you
did the following (timing is critical):
1. Short swipe from nav region towards HintState, but not far enough or
   fast enough to commit before letting go; this cancels the state
   animation, returning towards Normal (but, crucially, StateManager
   still has state set as Hint)
2. While previous animation is animating back to Normal, swipe up again,
   but this time faster/farther to actually reach Hint; this time, the
   animation does go towards Hint, but gets stuck there. The reason it
   gets stuck is because StateManager thinks we're already in Hint from
   step 1, so doesn't call onStateTransitionEnd(Hint) in step 2. Thus,
   we never get QuickstepLauncher#onStateSetEnd(Hint), which is what we
   rely on to return to Normal.

The simple fix is to prevent the second swipe in the first place.

Test: short swipe followed immediately by fast fling from nav region on home successfully stays in Normal state intead of getting stuck in HintState
Test:
NexusLauncherOutOfProcTests: com.google.android.apps.nexuslauncher.TaplTestsNexus
Fixes: 228276181
Change-Id: I54c371c8518a9a220e75c98003331b552d8bf8af