Convert Collection adapter to Kotlin (#1485)

* Consolidate contract utils

* Rename .java to .kt

* Convert CollectionJsonAdapter to Kotlin

* Lift factory into companion object

* Update moshi/src/main/java/com/squareup/moshi/internal/Util.kt

moshi/japicmp/build.gradle.kts

@@ -30,6 +30,9 @@ val japicmp = tasks.register<JapicmpTask>("japicmp") {
     "com.squareup.moshi.internal.NullSafeJsonAdapter", // Internal.
     "com.squareup.moshi.internal.Util" // Internal.
   )
+  fieldExcludes = listOf(
+    "com.squareup.moshi.CollectionJsonAdapter#FACTORY" // False-positive, class is not public anyway
+  )
 }
 
 tasks.named("check").configure {