fix: remove hardcoded android path

[szymonrybczak]

Summary:

By using ctx.project.android?.sourceDir clean command will work when Android project is under different path.

Test Plan:

1. Clone the repository and do all the required steps from the Contributing guide
2. Change android folder name and specify in react-native.config.js new path.
3. Run this command:

node /path/to/react-native-cli/packages/cli/build/bin.js clean

Choose android cache to clean, and command should be executed in correct place.

Checklist

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

[szymonrybczak]

While creating this PR I find out that we're not dynamically finding Android project if it doesn't exists under default path:

cli/packages/cli-platform-android/src/config/findAndroidDir.ts

Lines 12 to 18 in aebdd0a

	export default function findAndroidDir(root: string) {
	if (fs.existsSync(path.join(root, 'android'))) {
	return 'android';
	}
	return null;
	}

(in iOS part we're looking for Podfile and base on this location we're assuming iOS project location), should we do the same for Android? cc. @tido64 @cortinico If yes, what files should we look at?

[tido64]

(in iOS part we're looking for Podfile and base on this location we're assuming iOS project location), should we do the same for Android? cc. @tido64 @cortinico If yes, what files should we look at?

build.gradle is a good file to look for. @cortinico correct me if I'm wrong. I wouldn't look too deep for it though, you'll just risk false positives. In the repositories I have been in, I've only seen /android/build.gradle or /build.gradle.