[FRE][Android] Implement sign-in without sync from welcome screen

This CL implements the sign-in without sync from the welcome screen
when the native libraries and policy are loaded, when they are not
loaded, a spinner will be shown during the waiting time.

Demo: crbug.com/1227313#c4
Bug: 1227313, 1242899
Change-Id: Ic8305034afe5ff73a8392c3591ad335a5e587cf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3118145
Reviewed-by: Sky Malice <skym@chromium.org>
Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#920225}