Refactor AppListRepositoryImpl to add caching.

This change introduces `AppListRepositoryImplHelper` to contain the
original logic. `AppListRepositoryImpl` now wraps this helper and adds a
caching layer using `ConcurrentHashMap` and `Deferred` for `loadApps`
and `loadHomeOrLauncherPackages`, controlled by a `useCaching` flag.

Bug: 439793151
Flag: EXEMPT bug fix
Test: manual + presubmit
Change-Id: Ib92b9d60f596c1032549746c37fb29429eb03af8