Fix wrong Choreographer used for setting vsyncId in cross-activity predictive back

The constructor of CrossActivityBackAnimation is not called from the wmshell.main thread. Therefore we should not set the Choreographer from the constructor.

Bug: 343039961
Flag: com.android.window.flags.predictive_back_system_anims
Test: Manual, i.e. logging Choreographer instances and veryfying that they're accessed from wmshell.main thread. Also verified in perfetto that traces look smooth now.
Change-Id: Iead236279ea0f46acde23f16bb230ff324f453f3