Cleanup unused TestApi in TaskFragmentOrganizer

The callback handling is moved to SplitController#onTransactionReady, no
longer need those TestApi.

This is different from the merged-in cl for CTS compatibility in the
current release.

Bug: 240519866
Test: pass existing
Change-Id: I66ddd51c94003254001436ff0505dde3b26d0437
Merged-In: I66ddd51c94003254001436ff0505dde3b26d0437