fix: check for main manifest if multiple are present

[WoLewicki]

Summary:

This PR fixes a newly introduced (#1967) problem that having multiple build flavors in app causes the findManifest script to take the first found one. We think it seems more reasonable to take the one from main folder, if present. It fixes the error in Expensify app, where e2e manifest is the first found one, and the MainActivity is present in the main folder's AndroidManifest.xml. It throws such error then:

error Main activity not found in /Users/.../android/app/src/e2e/AndroidManifest.xml.

Test Plan:

Make a simple app with activity tag inside main AndroidManifest.xml and add a directory e.g. e2e with other AndroidManifest.xml, which should have the activity merged from the main. It will cause the error on gradle sync in RN 0.73.x

Checklist

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

[WoLewicki]

Gradle has a mechanism of merging the manifests, so if we could somehow reuse it, then it would most probably be a more general fix for weird package structures.

[szymonrybczak]

Thank you! 🙌 Mind adding tests? 🙏

[j-piasecki]

I've added a test case resembling a setup that's the reason for this PR. LMK if it's enough or if you want to cover other scenarios as well.