dPWA: Read is_placeholder info from web_app DB instead of external prefs

This CL contains the following:
1. Helper functions in the WebAppRegistrar to obtain is_placeholder info
per app_id and per install_url from the web_app DB.
2. A feature flag to gate the reading from the pref and the web_app DB
as required.
3. All calls to external prefs changed to WebAppRegistrar.
4. All tests referring to functionalities changed now run twice, once
using the external prefs and once using the registrar.
5. Removing the source from the config map as part of
WebApp::RemoveSource().

DD: go/external_web_app_prefs_migration

Bug: 1029410, 1321170
Change-Id: Ibab7ef7fb4e8dd442ff3d811a6ca26a930051ad7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3649331
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1007457}