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

<!--
Thank you for sending the PR! We appreciate you spending the time to work on these changes.
Help us understand your motivation by explaining why you decided to make this change.

If this PR fixes an issue, type "Fixes #issueNumber" to automatically close the issue when the PR is merged.

_Pull requests that expand test coverage are more likely to get reviewed. Add a test case whenever possible!_
-->
Pull Request resolved: https://github.com/facebook/react-native/pull/21782

Differential Revision: D12854056

Pulled By: hramos

fbshipit-source-id: d1e6a395e762cc4a4f133977d54d0f469fa66b8c

react.gradle

@@ -127,7 +127,7 @@ afterEvaluate {
                 }
 
                 // Workaround for Android Gradle Plugin 3.2+ new asset directory
-                into ("merged_assets/${targetPath}/merge${targetName}Assets/out") {
+                into ("merged_assets/${variant.name}/merge${targetName}Assets/out") {
                     from jsBundleDir
                 }
             }