Rename Function.java to .kt

In preparation for its conversion to Kotlin.

Test: ./gradlew bOS
Change-Id: Ia1f8f7f497b338ad80d2649584d47f025e39e5d5
diff --git a/core/core/src/main/java/androidx/core/util/Function.java b/core/core/src/main/java/androidx/core/util/Function.kt
similarity index 100%
rename from core/core/src/main/java/androidx/core/util/Function.java
rename to core/core/src/main/java/androidx/core/util/Function.kt