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Cryptohome already supports starting/ending fingerprint auth session
(i.e. preparing biometrics daemon for upcoming fingerprint scan / back
to normal mode).

This change enables ExtendedAuthenticator to start fingerprint auth
session so that ExtendedAuthenticator can be used to do fingerprint
auth.

The reason why fingerprint auth needs starting/ending auth session is
because it is expensive to switch mode of the fingerprint MCU. The
client should only end fingerprint auth session when no more fingerprint
retries are expected.

The reason why we are doing this by calling cryptohome, instead of
calling biometrics daemon, is that we want cryptohome to control all
authentication mechanisms, and there's a generalized "auth session"
project in cryptohome.

Bug: b:156258540, b:144861739
Change-Id: I767b2aff9791c838674d8ace8ab8d4494f69e603
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364894
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800758}