Fix blink when docking a window.

We need to be very precise about removing the old window when the new
one is shown. The right moment is immediately after the first frame
of the new window entrance animation gets commited. This requires
more infrastructure and flags, rather than depending on the old ones
and hoping that they will match our needs.

Bug: 25075151

Change-Id: I0fdbf38d0915ded3300d75fc7268e619b574bcf5