[iOS] Avoid signing out the supervised user when turning sync on

Before this CL, and as explained in crbug/1408071, the authentication
flow would sign the user out and back in when they would enable sync
for a supervised account. The bug was visible when run with Tangible
Sync as the TangibleSyncMediator would stop if the primary account
was cleared.

This CL addressed both issues as they both lead to this bug. It also
changes the FirstRunTwoStepsTestCase earl grey test suite to use
the tangible sync version of the UIs. This is needed as the test that
identified the bug is |testWithUMACheckedAndSigninSupervised|.

Note: The bug in TangibleSyncMediator would not appear with the current
change in authentication_flow.mm, however we believe that the current
behavior in TangibleSyncMediator is a bug as the authentication flow
may in some cases change the primary account. We preferred thus to
address the bug TangibleSyncMediator in the same CL.

Bug: 1408071, 1407658
Change-Id: I25cf2b0cff347c3e61b43f7f64999d1efcb8b6dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4173852
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Reviewed-by: James Lee <ljjlee@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1093337}