Fix SpringAnimation in Behavior when current value is set.

Fixes regression introduced by d489f2f6549a86b3949004d1c8ec68487fc2adb7.
We now only avoid running the animation if the Behavior wasn't already
mid-animation. This ensure correct behavior for SpringAnimation, and
also correct state for 'running'.

Task-number: QTBUG-21549
Change-Id: I0f97813294cca22fb7a273e222fa0549c6de9ca7
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>