Make sourcemaps script compatible with flavors

[louiszawadzki]

What does this PR do?

Make sourcemaps script compatible with builds using flavors for RN >=0.71 (cf #528).

Since RN 0.71, the path are no longer defined in the react.gradle file but in the react-native-gradle-plugin. In the plugin, the targetPath is variant.name and no longer variant.dirName.

Additional Notes

There is a other possibility for when RN >= 0.71 to get the sourcemaps and bundle path as attributes of the bundle task.
Even if this could potentially be more stable in the long run, I feel it could make the script more complex and in the end harder to maintain.
The proposed fix is a bit simpler and has less potential to break on other versions of RN.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change

[louiszawadzki]

targetName is the same for all cases, but I prefer to have it defined in here to have a more consistent scope in each if condition.
But I'm not against moving it to the top of the getReactConfig function if that makes more sense.