Fix incorrect merged asset path with flavor for Android Gradle Plugin 3.2. (#21782)

Summary:
When we use product flavor (e.g. develop/production) with Android Gradle Plugin 3.2,
javascript bundle is not copied due to that merged asset path point to incorrect location.

related #21408

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Pull Request resolved: https://github.com/facebook/react-native/pull/21782

Differential Revision: D12854056

Pulled By: hramos

fbshipit-source-id: d1e6a395e762cc4a4f133977d54d0f469fa66b8c