Fix a flicker when opening app again quickly while it's exiting

If the app is waiting for an opening animation with a dummy placeholder,
we need to skip the surface placement (in addition to the animateLocked).

Also, when animating is changing from exiting to entering, the mAnimating
flag needs to be cleared until the new animation starts. This prevents the
surface placement to place it wrong before the new animaition starts.

bug: 28599295
bug: 27742244
Change-Id: I26f0ead80ee9993a6c766ae8686ab11d1729519c