[ios] Change API of SignoutActionSheetCoordinator

Pass the completion block in the initializer instead of making it
a required property that must be set after calling the initializer.

Change client code to adapt to the new API, and when neceessary use
an helper method to simplify the callback implementation (as blocks
should generally just call a single method).

Bug: none
Change-Id: Ic9d6f4dd8b4c82ed976bb8d44717056905d25321
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6415650
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440175}