Fix modifying NBP callbacks in the callback

As callbacks can themselves request rebinds, they were modifying
NotifBindPipeline's callback list while it was still resolving the
previous pipeline completion. By marshalling all callbacks into a
scratch list and then iterating, we solve this issue.

Bug: 112656837
Test: atest NotifBindPipelineTest
Test: Notify apk group cancel test
Change-Id: I5a528d65a683a070c31b04da80b3a83d00dbce49