Make sure the app can draw a frame before unlocking

- The mechanism to stop windows drawing while window animator was
animating was somehow flaky. It relied on the fact that the client
would call relayout() whenever the animating state changed. This is
mostly the case, but not for lockscreen animations. Instead, we now
use a push model, where window manager tells the app that the state
has changed.
- In addition, it only stopped drawing if that window was animating,
but then only resumed drawing after all windows have finished
animating. Now, we do this per window, so we only stop drawing for
windows that are currently animating.
- We resume the top activity now at the very beginning of the
unlocking sequence. This gives the app a chance to draw a frame
before the user sees anything. If it's to slow, then we just use the
outdated framebuffer.

Bug: 19964562
Change-Id: Ifef8abd189a3146d854b81b9b948861e4d38c155