fix validate-android-test-env.sh (#22961)

Summary:
fix validate-android-test-env.sh to not parse .gradle files for build tools versions, because it was removed.

Changelog:
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[CI] [Changed] - fix validate-android-test-env.sh to not try parse .gradle files
Pull Request resolved: https://github.com/facebook/react-native/pull/22961

Differential Revision: D13655750

Pulled By: hramos

fbshipit-source-id: 1accb6ac7a7c74ab335dd92977b0b090820a622c