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This CL adds a UI placeholder for going incognito mode from MICE
account picker dialog.

The process of going incognito gated behind the incognito interstitial
would be followed up in later CLs.

This CL also adds an instrumentation test to verify the button is
shown after the bottom sheet is expanded and update the existing unit
tests in AccountPickerMediatorTest.

Bug: 1103262
Change-Id: I3762747ebb7b9bb50ebc32447635daf858876336
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288498Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Alice Wang <aliceywang@chromium.org>
Commit-Queue: Rohit Agarwal <roagarwal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789904}