Correct android font weight

[lukewalczak]

Fixes the issue with no effect on Android when fontWeight value is numerical.
More about the issue you can read here

Going through the comments and potential solutions I've created a function for mapping a font to font-weight called correctTextFontWeight which has to be called from the application starting point (_src/index.js_)

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To test:

1. Open WP-Android app
2. Open PAGE draft
3. Expect correct font weight (thicker - 600) on template buttons texts

To test:

1. Open playground app on Android
2. Scroll to unsupported block
3. Expect correct font weight (thicker - 600) on template buttons texts

To test:

1. Open playground app on iOS
2. Expect app is not crashing

NOTE: I would like to add some unit test if you like that approach.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.