[dPWA] Refactor callback for testing to be an observer method

This CL removes SetRemoveManagementTypeCallbackForTesting() from
WebAppInstallFinalizer and puts it in WebAppInstallManagerObserver as an
OnWebAppSourceRemovedForTesting() observer method instead.

This is in preparation to remove uninstallation methods from
WebAppInstallFinalizer as the implementation is now a command in
WebAppUninstallCommand.

Bug: 1427340
Change-Id: Ifd358d883b9fa08147017619ef024af6e408875e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4551587
Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148347}