ADFA-5000 fix(update): drive OTA verify from poller so the dialog can't hang on 'verifying'

[luisguzman-adfa]

Check for updates could get stuck on "Verifying signature…" and never reach
Install. Leaving that state depended only on ACTION_DOWNLOAD_COMPLETE, which is
lost when the download finishes in the background, in a receiver-registration race,
or on OEMs that don't deliver it. The poller had already seen the download succeed
and stopped, so verification was never triggered and the dialog hung.

The fix drives the verify/install step from UpdateViewModel's poller (which reads
DownloadManager directly and fires even without the broadcast). The poller and the
broadcast now funnel into one idempotent handleDownloadComplete(), so verification
runs exactly once; the broadcast stays as a fast-path backup, and the signature
check moved off the main thread. Only UpdateViewModel and UpdateController change.

Testing: a fake OtaDownloadGateway returning SUCCESSFUL confirms the poller
reaches verification without a broadcast; on device, point the updater at a local
update.json + a self-signed build and background the app mid-download — it should
still reach Install.

Tested on device