Fix stopping process for fast tracks

Previously, the state of a fast track "wiggled" back and forth at the end.

Now it goes through these transitions:
  active -> stopping_1 -> stopping_2 -> stopped

This CL is only for fast tracks, and does not change how
normal tracks work.

Change-Id: Icc414f2b48c46dda63cfa6373ca22d033dd21cd4