We have a number of mechanisms (web_app::ExtensionIdsMap, various flags on BookmarkAppHelper) to control how default/local/no-network/policy installed apps work and are/aren't garbage collected. Many of these are raw boolean options or non-ergonomic.

We should:

 - use a better set of flags for default installed etc. which communicate the effect we want (e.g. don't sync, remove automatically)
 - consolidate auto removal code somehow