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Add the "BACK" button into the PIN keypad displayed on a user pod that
use the challenge-response authentication (a.k.a. smart card login).

Clicking this button allows to exit the PIN keypad and abort the
challenge-response authentication.

Before this CL, there was no way to abort the authentication besides
waiting some internal timeout (e.g., a few minutes for D-Bus calls) is
hit.

Bug: 983103
Test: start existing challenge-response user sign-in, check that the PIN keypad with the BACK button is shown, click BACK, check that the PIN keypad disappears
Change-Id: Ideb451820b40578f06e98db97fb259473118b047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1784616
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693686}