Add fallback values for promo menu

[eljamm]

Old Android WebView versions might not support calc or math functions properly, if at all, which makes the promo menu appear empty.

To solve this, either the user needs to update their WebView to the latest version or we can add fallback values as a catchall.

These are the tests I've done on my Android 8.1 device :

• WebView 70 + 1.7.16 release -> Empty Menu
• WebView 70 + 1.7.16 release (with fallbacks) -> Correct Menu
• WebView 104 + 1.7.16 release -> Correct Menu

I don't know if this fully fixes the empty menu issue, but for my device it appears to be the case.