feat: add androidx support

[erisu]

Motivation, Context, & Description

Adds AndroidX Support
Removes the need for the cordova-plugin-androidx plugin.

closes: #565

Testing

• npm t
• cordova platform add
• cordova build android

Checklist

• I've run the tests to see all new and existing tests pass
• 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-io]

Codecov Report

Merging #901 into master will decrease coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
- Coverage   66.17%   66.03%   -0.15%     
==========================================
  Files          19       19              
  Lines        1839     1843       +4     
==========================================
  Hits         1217     1217              
- Misses        622      626       +4

Impacted Files	Coverage Δ
...lates/cordova/lib/config/GradlePropertiesParser.js	76.66% <ø> (ø)	⬆️
bin/templates/cordova/lib/prepare.js	35.11% <0%> (-0.48%)	⬇️

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[knight9999]

This will be just fine.
By the way, because the test code /test/app/buld.gradle uses old com.android.support:appcompat-v7, it is better to add new test app for Android X (using androidx.appcompat:appcompat-v7).

[erisu]

@knight9999 I added the AndroidX Java unit tests.