Expose isUserKeyUnlocked as a TestAPI

This is needed to replace usage of the adb shell
command 'dumpsys mount' in some cts tests -to identify
if user key is locked- with a more reliable TestAPI

Bug: 173695151
Bug: 180328483
Test: N/A
Change-Id: Iebcc0080195b207f133e8d7186a098f39e137f42

core/api/test-current.txt

@@ -1614,6 +1614,7 @@ package android.os.storage {
   public class StorageManager {
     method @NonNull public static java.util.UUID convert(@NonNull String);
     method @NonNull public static String convert(@NonNull java.util.UUID);
+    method public static boolean isUserKeyUnlocked(int);
   }
 
   public final class StorageVolume implements android.os.Parcelable {

core/java/android/os/storage/StorageManager.java

@@ -1536,6 +1536,7 @@ public class StorageManager {
     }
 
     /** {@hide} */
+    @TestApi
     public static boolean isUserKeyUnlocked(int userId) {
         if (sStorageManager == null) {
             sStorageManager = IStorageManager.Stub