Factor hardcoded image paths out in RNTester/Image example, get rid of .ico file (#44415)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44415

# Changelog:
[Internal] -

RNTester Image example used hardcoded `https://www.facebook.com/favicon.ico`, which has an uncommon ICO format, for no good reason aside of just this image being served from `facebook.com`.

This diff:
* Replaces the ICO image with a PNG one (which is still served from `facebook.com`
* Factors out all the multiple hardcoded paths into constants, so that it's easier to make such changes in the future
* Changes another image to something that is a bit better on the eyes when severely downscaled

Reviewed By: christophpurrer

Differential Revision: D56978929

fbshipit-source-id: c627d1671c8cb66e9a78f4382faa56e539b2f7b3