cros: Correctly handle login state on local accounts policy change

Hide Gaia screen when switches from 0 to >0 users, and show the
account picker screen otherwise.

Also remove can_close_ param from ShowGaiaSignin - relies on the user
list now.

Bug: 647451
Change-Id: Iec4e8182b6e07393a10d2336a9818d7d1b2d0b48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035670
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743971}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/login/ui/lock_contents_view.cc[diff]
• ash/login/ui/lock_contents_view_unittest.cc[diff]
• ash/login/ui/login_auth_user_view.cc[diff]
• ash/login/ui/login_auth_user_view_unittest.cc[diff]
• ash/login/ui/login_expanded_public_account_view.cc[diff]
• ash/public/cpp/login_screen_client.h[diff]
• ash/shelf/login_shelf_view.cc[diff]
• chrome/browser/chromeos/login/saml/saml_browsertest.cc[diff]
• chrome/browser/chromeos/login/ui/fake_login_display_host.cc[diff]
• chrome/browser/chromeos/login/ui/fake_login_display_host.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host_mojo.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_mojo.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host_webui.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_webui.h[diff]
• chrome/browser/chromeos/login/ui/mock_login_display_host.h[diff]
• chrome/browser/chromeos/policy/device_login_screen_policy_browsertest.cc[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]