Make Environment.getProductDirectory() @TestApi

Tests should be allowed to know where the product directory is mounted.

Bug: 119049497
Test: atest SysConfigTest
Change-Id: I3f196a862ad10d21646d4d5f8e46f6eec0a3bf6c
diff --git a/core/java/android/os/Environment.java b/core/java/android/os/Environment.java
index 0c4a0b3e..55951ca 100644
--- a/core/java/android/os/Environment.java
+++ b/core/java/android/os/Environment.java
@@ -199,6 +199,7 @@
      * @hide
      */
     @SystemApi
+    @TestApi
     public static File getProductDirectory() {
         return DIR_PRODUCT_ROOT;
     }