Warn on Protocol-relative URLs

[david-allison]

Protocol relative URLs didn't work:

Fixes

Fixes #6102

Approach

We add a snackbar to warn the user and explain how to fix this.

In Anki, protocol-relative URLS are converted to https://
In AnkIDroid, they're converted to file://

file:// fails to load the image - we can't redirect from shouldInterceptRequest
Loading synchronously is not viable, so all we can do is use a snackbar.

How Has This Been Tested?

On my phone and API 19 emulator

⚠️ Issue: #7665

Learning

• HTTP 300s aren't supported by shouldInterceptRequest
• Section links seem broken on some phones.
• https://github.com/ankidroid/Anki-Android/wiki/FAQ#why-do-my-images-need-a-protocol

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
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• Strings resources: All strings added as resources are unique or contain a comment for seemingly duplicate strings to explain the difference between both resources

[mikehardy]

LGTM - what do you think, 2.14-able? This looks like a bug fix to me and is an area well known to you. Might cause a quick follow-on but I can put this out as a beta today

[david-allison]

@mikehardy It's a bugfix, hopefully non-functional, so 2.14 seems sensible