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Extend PinBackend to support the PIN auto submit functionality.

PIN auto submit just exposes the user's PIN length in local state
when its enabled. Normal users are prompted with a dialog in Settings
for their PIN. After checking that the PIN is correct, the length is
exposed. Managed users who cannot control the feature have their PIN
lengths collected upon a successful authentication attempt.

A new method - SetPinAutoSubmitEnabled - was added to allow Chrome OS
Settings to control the functionality directly. The basic operations
in PinBackend (Set, Remove, TryAuthenticate) were extended so that
they now call PIN auto submit subroutines to do additional steps that
are necessary for the new functionality. These methods were named
PinAutosubmitOnSet, PinAutosubmitOnRemove and PinAutosubmitOnTryAuth.

In order to extract the PIN length upon a successful authentication
attempt, the call to Cryptohome is now calling the new method
'OnCryptohomeAuthenticationResponse' first, instead of the
original caller.

All of the PIN auto submit subroutines are guarded by the feature
flag, which is disabled by default.

Unit tests were added to cover all the possible user flows.

Design Document
- https://goto.google.com/chromeos-oac-pin-authentication-refinements

Bug: 1107367, 1107367
Change-Id: Ie770727c73c9abece174a10fd8c618575f849b88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2306258
Commit-Queue: Renato Silva <rrsilva@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791405}