Fix build error for Android projects that use `apply plugin: "com.android.library"` (#22312)

Summary:
This PR allows Android projects that use `apply plugin: "com.android.library"` to build successfully. A recent regression caused by https://github.com/facebook/react-native/pull/20526 means that building one of these projects always fails with this error:

```
FAILURE: Build failed with an exception.

* Where:
Script '/Users/james/src/SampleLibrary/node_modules/react-native/react.gradle' line: 15

* What went wrong:
A problem occurred configuring project ':app'.
> Could not get unknown property 'applicationVariants' for object of type com.android.build.gradle.LibraryExtension.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 0s
```

This change updates `react.gradle` to cater to both application and library projects by selectively using `android.applicationVariants` or `android.libraryVariants`.

Fixes https://github.com/facebook/react-native/issues/22310.
Pull Request resolved: https://github.com/facebook/react-native/pull/22312

Differential Revision: D13373742

Pulled By: cpojer

fbshipit-source-id: 64c35ab7a6d5d0d840a43729123e70dd8e0d36e0