Fix flicker when interrupe back navigation transition.

For unity back navigation transitions, the system server forces the
original top window to participate in the predictive back transition.
If an interrupting transition occurs during the predictive back
transition, the animating target should also participate in the second
transition. This ensures that when the predictive back transition is
canceled in the Shell, the second transition can seamlessly continue
with the animating target, which is then reparented the surface back to
hierarchy after the second transition finishes.
Also prevents the pending predictive back transition from starting
after a transition finishes if another transition is already playing.

Flag: com.android.window.flags.unify_back_navigation_transition
Bug: 390345225
Test: press device edge quickly during an open transition to try to
trigger close transition, verify if predictive back transition has
triggerred, the transition can be canceled and default transition
handler can play the following transition without flicker/jumpcut.

Change-Id: I1fe5c2ea509f7000b15be5d69336a1c28d9e91ed