shimless_rma: Add feature flag for allowing permission policies

Add a feature flag `kShimlessDiagnosticsInstallRequestPermission`
which will control enabling permission policies in IWA for shimless
diagnostics app.

BUG: b:317466135, b:294815884
TEST: testing/xvfb.py out/Default/unit_tests --gtest_filter="*ChromeShimlessRma*"
Change-Id: I788af64cacd1af0488abf2b359b50831fd698fbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5307316
Commit-Queue: Ethan Cheng <yycheng@google.com>
Reviewed-by: Chung-sheng Wu <chungsheng@google.com>
Reviewed-by: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1284379}