Open up getCloudMediaProvider to mainline modules.

Bug: 220890919
Test: atest CtsPhotoPickerTest
Change-Id: I2933b622eede53f87baff94da12cc9b3cc532456

core/api/module-lib-current.txt

@@ -413,6 +413,7 @@ package android.os.storage {
 
   public class StorageManager {
     method public long computeStorageCacheBytes(@NonNull java.io.File);
+    method @Nullable public String getCloudMediaProvider();
     method public void notifyAppIoBlocked(@NonNull java.util.UUID, int, int, int);
     method public void notifyAppIoResumed(@NonNull java.util.UUID, int, int, int);
     method public void setCloudMediaProvider(@Nullable String);

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

@@ -3022,9 +3022,17 @@ public class StorageManager {
         }
     }
 
-    /** @hide */
-    @TestApi
+    /**
+     * Returns the authority of the current cloud media provider that was set by the
+     * {@link android.service.storage.ExternalStorageService} holding the
+     * {@link android.Manifest.permission#WRITE_MEDIA_STORAGE} permission via
+     * {@link #setCloudMediaProvider(String)}.
+     *
+     * @hide
+     */
     @Nullable
+    @TestApi
+    @SystemApi(client = SystemApi.Client.MODULE_LIBRARIES)
     public String getCloudMediaProvider() {
         try {
             return mStorageManager.getCloudMediaProvider();