Javascript API: Add ankiIsActiveNetworkMetered

[david-allison]

Purpose / Description

Allows for deck developers to stop loading heavy assets such as videos on LTE

Fixes

Fixes #6783

Approach

Add relevant JS API

How Has This Been Tested?

• Tested on my mobile - returns false when I'm on WiFi
  • Using https://github.com/ankidroid/Anki-Android/wiki/Development-Guide#html-javascript-inspection
  • AnkiDroidJS.ankiIsActiveNetworkMetered() //false;
• 🛑 Could a reviewer please test this on LTE. I'm paying £0.20/MB, and can't realistically use 4G (probably a good use-case for this API, but it'll cost me quite a lot of money to test this).

Learning (optional, can help others)

• JS API opens exciting possibilities
• https://developer.android.com/reference/android/net/ConnectivityManagerCompat#isActiveNetworkMetered() - doesn't say much
• https://stackoverflow.com/a/23879816
• https://developer.android.com/reference/androidx/core/net/ConnectivityManagerCompat#isActiveNetworkMetered(android.net.ConnectivityManager) - actually does exactly the same on API 16+.

This returns true for mobile data, can be true for WiFi as well if it's detected to be metered

Checklist

• You have not changed whitespace unnecessarily (it makes diffs hard to read)
• You have a descriptive commit message with a short title (first line, max 50 chars).
• Your code follows the style of the project (e.g. never omit braces in if statements)
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code

[mikehardy]

I help maintain react-native-netinfo so I've certainly seen these APIs before, looks right to me, with the compat as well etc