Cancel unlock animation if power button pressed in tablet mode

The normal power button behavior in tablet mode is to lock the device
and turn off the screen. However, if the power button is pressed during
the unlock animation, for example, after a successful fingerprint scan,
then session manager will ignore the lock attempt as the session would
already be considered locked during the animation, leading to an
unlocked session behind a turned off screen.

This CL resets the lock screen and cancels the unlock animation if the
power button was pressed during the unlock animation.

Bug: b/234146799
Change-Id: I1ed35d50ef3df067873f75a912ae83087f88df1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3748102
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Elie Maamari <emaamari@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1026225}