app.uninstallable is only used in AppDetails. It is only set when
generating App instances from installed APKs for the swap stuff. Since it
is initialized to false and used as !app.uninstallable, it is always true
when used. So it was doing nothing.
This needs to be thought out more so this is not entirely complete for
#628. AppDetails needs to know whether its a system app to provide proper
feedback and swap needs to know whether its a system app with an update
installed, otherwise it should ignore it.
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