Add support for carrier "associated" apps.

The platform currently supports the notion of default carrier apps.
These apps are set to DISABLED_UNTIL_USED until a SIM is inserted
which grants them carrier privileges, at which point they are enabled.
Apps are not touched if they have been updated from the version on
/system or if their state has been modified externally (e.g. by the
user).

This CL extends this notion to associated apps, which may not have
carrier privileges themselves, but should be enabled/disabled
alongside a particular carrier app. This should include helper apps
that should not be visible to users who don't use the given carrier
unless the user explicitly enables the app.

As additional protection, we add a check to ensure that we never
disable apps after the first time we've run. Since we need to store
this information in secure settings, we also move the call site from
PackageManagerService#main() to PackageManagerService#systemReady(),
which enables use of secure settings but still occurs before
third-party apps can be started.

Bug: 30141427
Change-Id: Iee72ba4e70e5ca97999c9147a65af82c670a23e8