Call stopService to ensure notification goes away. Always show download notifications.

Addresses a bug found in MR !273 whereby removing `stopForeground` results
in a persistent "Downloading ..." notification even though it was cancelled.

In the process of doing this, it also addresses / Fixes #621 by ensuring
that all downloads of apks are done in a foreground service, regardless
of the preference used for foreground updater service updating.
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