[Autofill Assistant] Integrate Payment Request.

This patch is the first step to integrate the payment request UI into
the autofill assistant sheet.

The patch has and leaves a few rough edges but we'll deal with that
piece by piece. In particular, the dismiss logic needs to be handled
differently, so does the error view logic.

Note: The DimmingDialog is reverted to package default visibility since
the dependency from autofill assistant is removed here.

Bug: 806868
Change-Id: I25b83b804e13bbdec5d2a1688dd01cd085615b38
Reviewed-on: https://chromium-review.googlesource.com/c/1324451
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606870}
8 files changed