[Signin][Android] Revise checks in SigninFirstRunFragmentRenderTest

Replaces signin_fre_selected_account !isShown() polling with an
Espresso check for fre_native_and_policy_load_progress_spinner not
being displayed.

There are multiple reasons signin_fre_selected_account can be hidden
(for example, maybe native isn't initialized yet). Thus, it is safer
to verify that fre_native_and_policy_load_progress_spinner is hidden.

This is a preparation for https://crrev.com/c/3778377, which hits
the issue with signin_fre_selected_account check.

Bug: 1343837
Change-Id: I2ca22190bc0512c6f48eaff2b2d15c0d55727fa8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3779766
Commit-Queue: Tanmoy Mollik <triploblastic@chromium.org>
Auto-Submit: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Tanmoy Mollik <triploblastic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1028663}