fix: Use custom app name when getting build.gradle

[TMisiukiewicz]

Summary:

It cannot access package name from build.gradle when the module is named differently than app. This PR is fixing it.

Closes #2516

Test Plan:

1. Init fresh project
2. Change android/app folder name to android/test
3. Create react-native.config.js file with the following content:

module.exports = {
  project: {
    android: {
      appName: 'test',
    },
  },
};

4. Run android command and verify the error about missing package name is not thrown

Checklist

• Documentation is up to date to reflect these changes.
• Follows commit message convention described in CONTRIBUTING.md

[cortinico]

3. Create react-native.config.js file with the following content:

Is this documented somewhere or is it a new config?

[szymonrybczak]

Is this documented somewhere or is it a new config?

If you mean the property appName -- yes it was added a while ago and it's document here.

[cortinico]

Is this documented somewhere or is it a new config?

If you mean the property appName -- yes it was added a while ago and it's document here.

yup that one 👍 thank you

[szymonrybczak]

Let's fix CI failures and it's good to go for me!