[IdentityManager] Expose unconsented primary account

This CL implements API that conceptually belongs to IdentityManager and
that was previosly implemented in multiple places:
 - signin_ui_util::GetAccountsForDicePromos(),
 - syncer::DetermineAccountToUse().

Bug: 983182
Change-Id: I17282c87f62058c10830aa7b79648b40cbb66ab8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1698363
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678228}