policy: Support BlockExternalExtensions on desktop Android

Mark policy as future_on, as is our convention. Enable the unit tests
for ExternalProviderImpl on desktop Android for coverage.

Create a test directory "external_app" to hold the existing test CRX,
which is an app, which isn't supported by desktop Android. Add a new
test extension under "external_extension" to use for these tests.

Bug: 466062003
Change-Id: Icc1f47a08c97b68eb27fa66f0b1681178edcd116
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7227747
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Michael Wojcicka <mwoj@google.com>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1554833}