Convert ImageSizeMap to Kotlin

[nbradbury]

Closes CMM-351

This PR converts ImageSizeMap to Kotlin and addresses various nullability issues. I probably could've done this as part of #21877 but that was already quite large so I opted to handle it separately.

The purpose of this class is to store a hashmap of normalized image urls and their sizes. This is used by ReaderPostRenderer when laying out images.

To test, smoke test the Reader and verify that images appear as expected (or, at least, appear the same as they do in trunk).

[dangermattic]

	1 Warning
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[wpmobilebot]

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	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr21888-56b4f9d
	Commit	56b4f9d
	Direct Download	jetpack-prototype-build-pr21888-56b4f9d.apk

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr21888-56b4f9d
	Commit	56b4f9d
	Direct Download	wordpress-prototype-build-pr21888-56b4f9d.apk

Note: Google Login is not supported on these builds.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.63%. Comparing base (d64be53) to head (56b4f9d).
Report is 1 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk   #21888      +/-   ##
==========================================
+ Coverage   39.62%   39.63%   +0.01%     
==========================================
  Files        2125     2124       -1     
  Lines       99254    99218      -36     
  Branches    15253    15243      -10     
==========================================
  Hits        39330    39330              
+ Misses      56463    56427      -36     
  Partials     3461     3461              

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[adalpari]

LGTM!