Fixed deadlock between AM and WM when animating pinned stack.

In Ie18cc04ceeabd94093017f79676253e1189f5249 we delay ending the
pinned stack animation until app transition is done. However, the
signal that app transition is done comes in with the window manager
lock held and the we call into AM to do some additional work which
requires AM lock to be aquired. Since AM depends on WM we shouldn't
be holding the WM lock while calling into AM since AM can call into
WM with its lock held. We now post a runnable to finish the pinned
stack animation without the WM lock held.

Bug: 28564514
Change-Id: I8450123e992e569a74609883de810cdc611ced81