[iOS][UNOp2] Add EGTests/unittests for Upgrade promo metrics and prefs

This patch adds test for crrev.com/c/4925234. Related to:
crbug.com/1491096.

Unittests in TwoScreensSigninCoordinatorTest makes sure no upgrade
promo metrics/prefs are updated when TwoScreensSigninCoordinator is
created with another the access point than ACCESS_POINT_SIGNIN_PROMO.

EGTests in UpgradeSigninPromoTestCase checks that the upgrade promo
updates upgrade promo metrics/prefs, with
kReplaceSyncPromosWithSignInPromos flag enabled.

This patch had to move constants from user_signin_constants.h and
upgrade_signin_logger.mm to signin_constants.h, so they can be used
in unittests and EGTests.

Also `-[ChromeEarlGrey userDefaultObjectForKey:]` is added.

Bug: 1470750
Change-Id: I0e25228b595fb4325176dd54667853f7043631af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4931414
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1210095}