Add infrastructure to manipulate AutofillTestClient post initialization

Having this additional infrastructure will allow test classes derived
from `AutofillMetricsTestBase` to manipulate the client after it is
initialized.

This allows to efficiently inject mocks into the client before it is
used to construct the AutofillDriver and such.

Change-Id: I5b8237f22a51bb7fdd4c7b462847c7bcd91b73bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6973472
Commit-Queue: Matthias Körber <koerber@google.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519907}