JS module loading for older Androids (fixes #9311)

[nihil-admirari]

Purpose / Description

JS modules must have text/javascript MIME, otherwise they won't load: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules.

Fixes

Fixes #9311.

Approach

guessMimeType() works on Android 10+. MIME type must be set explicitly on older Androids.

How Has This Been Tested?

Android 8.0 and 13.0 in emulator.

Android 8 on Samsung T585.

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[nihil-admirari]

Rebased on current main, otherwise identical to #13812.

[nihil-admirari]

Tested on Samsung T585; #10175 is not reproducible.

[mikehardy]

I remember looking at this before in the superceded PR, I was fine with it then.
I rebased it against current main and it is clean, builds fine. LGTM, thank you and thanks for your patience!

[github-actions]

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