Partially addresses: #1485
Show the microG installer whenever the companion app is present instead
of relying on the misleading vending_apps_install metadata flag, and warn
users to enable it from microG settings before use. The dialog also offers
a shortcut to open microG settings.
microG only exposes the top-level settings activity; the App Installer
Settings screen is a Navigation fragment with no deep link, so the dialog
guides users through the remaining steps.
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