Ensure session listener is only registered once on init

With com.android.wm.shell.fix_missing_user_change_callbacks,
UserChangeListener is invoked immediately on registration with the
current user, so to avoid a double session listener registration skip
the first one if the flag is enabled.

Flag: com.android.wm.shell.fix_missing_user_change_callbacks
Bug: 406255019
Bug: 406463255
Test: atest PipControllerTest
Change-Id: I611b58c9d1aea11bc57347d555ce3de7c8233fe4