Remove SurfaceFreezer leash when unfreeze

Before, after unfreeze, the leash will not be removed until its parent
is gone, which may stay for a while. Now, it should be cleaned up
immediately.

Bug: 196173550
Test: test with demo app with ag/15559306
Change-Id: Ie21019bddbdaa6fddc351d07cc372f621ae5176e