Raise the minimum SDK for PDF Viewer from Android 8 to at least Android 9 or 10

[girlbossceo]

Ideally it should be Android 10 or 11 because the minimum WebView version allowed is 89, and Chromium 89 came out in March 2021 but that may be too aggressive.

Either way, Android 8 is way too low which came out in August 2017 and the decision for choosing this predates current project branding. People are downloading PDF Viewer from Play Store not realising they need a modern WebView and giving it a bad rating just because of that. Raising the minimum SDK prevents people from getting the app and giving us a bad review.

Alternatively, the minimum SDK could be high on the Play build variant in gradle, and slightly lower for non-Play build (GrapheneOS) so OS's that are fairly old can still use PDF Viewer if their OS keeps the WebView up to date.