Do not remove file_cache when exiting demo mode

When clearing /data/preloads directory, keep files in file_cache directory.
The content there will only be removed if systems runs low on storage.

Test: manual + RetailDemoModeServiceTest
Bug: 31008665
Change-Id: Id335d84940ee829617986f5345f169271a333de1
diff --git a/core/java/android/os/Environment.java b/core/java/android/os/Environment.java
index 4616af8..8d9ceb6 100644
--- a/core/java/android/os/Environment.java
+++ b/core/java/android/os/Environment.java
@@ -380,6 +380,24 @@
     }
 
     /**
+     * Returns location of preloaded cache directory for package name
+     * @see #getDataPreloadsDirectory()
+     * {@hide}
+     */
+    public static File getDataPreloadsFileCacheDirectory(String packageName) {
+        return new File(getDataPreloadsFileCacheDirectory(), packageName);
+    }
+
+    /**
+     * Returns location of preloaded cache directory.
+     * @see #getDataPreloadsDirectory()
+     * {@hide}
+     */
+    public static File getDataPreloadsFileCacheDirectory() {
+        return new File(getDataPreloadsDirectory(), "file_cache");
+    }
+
+    /**
      * Return the primary shared/external storage directory. This directory may
      * not currently be accessible if it has been mounted by the user on their
      * computer, has been removed from the device, or some other problem has