feat: add backwards compatibility mode for WebViewAssetLoader

[NiklasMerz]

Platforms affected

Android

Motivation and Context

As we discussed on the list we might need a fallback for the new changes in #1137

With this PR you can fall back to using files instead of https://localhost with this preference:
<preference name="AndroidInsecureFileModeEnabled" value="true" />

Documentation: apache/cordova-docs#1172

Testing

Manual

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #1222 (7ff9638) into master (f9e8ce8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1222   +/-   ##
=======================================
  Coverage   71.97%   71.97%           
=======================================
  Files          21       21           
  Lines        1695     1695           
=======================================
  Hits         1220     1220           
  Misses        475      475           

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[jcesarmobile]

tested and works fine, thanks!