Simplify the association of shared starting window

The issue case is launching an existing embedded activity with
clear-task and then a new activity is started in a new task fragment.
E.g. resume left panel -> finish right panel -> create right panel.
And the transition starts until the right panel is drawn instead of
the drawn starting window. That would feel a noticeable delay.

This is mainly for showing starting window on a task with embedded
activities, but also generalizes the common cases.

1. Move mSharedStartingData from ActivityRecord to Task.
   So if there is a new embedded activity launched in the task, it
   doesn't need to take care whether/when to assign the field. And
   no need to traverse all embedded activities in the task to make
   sure the shared starting data is up to date.

2. Move startingDisplayed from ActivityRecord to StartingData.
   The state of flag startingDisplayed is covered by the lifetime
   of StartingData. So it no longer needs to reset the flag.
   This also provides the ability to know whether the starting
   window is drawn or not without based on an activity or window.

With above changes, any activity without owned starting window can
know if the shared starting window is ready to execute transition.

Also use a non-empty reference activity in createRemoteAnimationTarget,
so the OPENING transition animation won't be skipped (the right panel
is the top non-finishing activity but it hasn't a child window yet).

Bug: 255688302
Test: atest ActivityRecordTests#testStartingWindowInTaskFragment

Change-Id: I7adeed8801ea3a837afa3d46f1975cf6c5b3f99a