A download row previously stopped at COMPLETED and never reflected whether the
app actually installed. Add INSTALLING/INSTALLED states driven by a central
installer-event observer in DownloadHelper:
- COMPLETED -> INSTALLING on the installer's first progress event
- -> INSTALLED on success (row kept so the APK can still be exported)
- a failed install reverts INSTALLING -> COMPLETED so it can be re-installed
without re-downloading
Consumers that branched on COMPLETED are updated (App Details state, MicroG
status mapping, Downloads list icon). downloadStatus is stored as TEXT so no
schema migration is needed.
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