Use best icon resolution in index for APK extracted icons

Currently, all APK extracted icons link to https://f-droid.org/repo/icons/ where the resolution is 48x48. Instead, we should use https://f-droid.org/repo/icons-640/ where the resolution is 192x192.

Example:

Otherwise, on high resolution displays or when app icons are shown bigger, they look blurry, because they need to be scaled up.

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