Revert "Run callback registration on the worker provided by the client."

This reverts commit 1b917593.

Reason for revert: Causes NPE in internalRegisterCallback, probably a race condition.

Bug: b/369292752
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:673f9e5c8550e1205749ab8e8f4189e4778f6d03)
Merged-In: Ic58e7eee51e0f3a15d62ca6e5db85f1b291bd865
Change-Id: Ic58e7eee51e0f3a15d62ca6e5db85f1b291bd865