[dPWA] Refactoring relative manifest id to absolute id.

This patch refactors the manifest parser & id handling to use the
computed manifest_id defined in the spec, which simplifies a lot of
code. This patch:
- Makes changes to the ManifestParser to output the id as a full url.
- Always populates the manifest_id web app representations.
- Fixes a variety of tests, including some small mistakes in the sub app
  tests that are hopefully minor.
- Adds a number of checks to ensure that things are created correctly.
- Updates the web_app_helper methods appropriately.

This change should have no functional impact.

Bug: 1231765,1442799,b/280606515
Change-Id: I6d69e2d94af9278d796df4efb3e275416e4ad59e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4515997
Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1144248}