Made "getPlainText" a public method for use in native code

[crutchcorn]

Currently, there is no easy way to access a secure token from the native code of an app. However, there are instances where this may be useful.

This PR makes the getPlainText method public so that we're able to use it in Java native interop code:

public class GitModule extends ReactContextBaseJavaModule {
    private static ReactApplicationContext reactContext;

    GitModule(ReactApplicationContext context) {
        super(context);
        reactContext = context;
    }

    @ReactMethod
    public void clone(
        String uri,
        String path,
        Promise promise
    ) {

        RNSecureKeyStoreModule keyStoreModule = new RNSecureKeyStoreModule(reactContext);

        String ghToken = "";
        try {
            ghToken = keyStoreModule.getPlainText("ghToken");
        } catch (Throwable e) {
            e.printStackTrace();
            promise.reject(e);
        }
    }
}

This would be the answered question for #35