Fix obsolete usages in build.gradle (#1168)

Summary:
Remove warning `'variant.getJavaCompile()' is obsolete and has been replaced with 'variant.getJavaCompileProvider()'.`

## Changelog

- swithed from `variant.getJavaCompile()` to `variant.getJavaCompileProvider()`
Pull Request resolved: https://github.com/facebook/flipper/pull/1168

Test Plan: n/a

Reviewed By: passy

Differential Revision: D21618883

Pulled By: cekkaewnumchai

fbshipit-source-id: 522fe5847b074836cc36a9e1b9a0487b044cf555

react-native/react-native-flipper/android/build.gradle

@@ -139,8 +139,10 @@ afterEvaluate { project ->
 
     android.libraryVariants.all { variant ->
         def name = variant.name.capitalize()
-        task "jar${name}"(type: Jar, dependsOn: variant.javaCompile) {
-            from variant.javaCompile.destinationDir
+        def javaCompileTask = variant.javaCompileProvider.get()
+        
+        task "jar${name}"(type: Jar, dependsOn: javaCompileTask) {
+            from javaCompileTask.destinationDir
         }
     }