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: #21782

Differential Revision: D12854056

Pulled By: hramos

fbshipit-source-id: d1e6a395e762cc4a4f133977d54d0f469fa66b8c

Author: yatatsu

react.gradle

@@ -132,7 +132,7 @@ 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
                 }
             }