Update direct boot related documentation.

Also hide a few APIs as requested by council.  Add a method to
easily determine if a given File would already be encrypted at rest
by the OS.

Bug: 27531029
Change-Id: Icad5f1cd56411ad3ac707db85fd7449acdcc4b94
diff --git a/core/java/android/os/Environment.java b/core/java/android/os/Environment.java
index 307bd2d..f6e8940 100644
--- a/core/java/android/os/Environment.java
+++ b/core/java/android/os/Environment.java
@@ -34,6 +34,7 @@
     private static final String ENV_EXTERNAL_STORAGE = "EXTERNAL_STORAGE";
     private static final String ENV_ANDROID_ROOT = "ANDROID_ROOT";
     private static final String ENV_ANDROID_DATA = "ANDROID_DATA";
+    private static final String ENV_ANDROID_EXPAND = "ANDROID_EXPAND";
     private static final String ENV_ANDROID_STORAGE = "ANDROID_STORAGE";
     private static final String ENV_DOWNLOAD_CACHE = "DOWNLOAD_CACHE";
     private static final String ENV_OEM_ROOT = "OEM_ROOT";
@@ -54,6 +55,7 @@
 
     private static final File DIR_ANDROID_ROOT = getDirectory(ENV_ANDROID_ROOT, "/system");
     private static final File DIR_ANDROID_DATA = getDirectory(ENV_ANDROID_DATA, "/data");
+    private static final File DIR_ANDROID_EXPAND = getDirectory(ENV_ANDROID_EXPAND, "/mnt/expand");
     private static final File DIR_ANDROID_STORAGE = getDirectory(ENV_ANDROID_STORAGE, "/storage");
     private static final File DIR_DOWNLOAD_CACHE = getDirectory(ENV_DOWNLOAD_CACHE, "/cache");
     private static final File DIR_OEM_ROOT = getDirectory(ENV_OEM_ROOT, "/oem");
@@ -229,6 +231,11 @@
     }
 
     /** {@hide} */
+    public static File getExpandDirectory() {
+        return DIR_ANDROID_EXPAND;
+    }
+
+    /** {@hide} */
     public static File getDataSystemDirectory() {
         return new File(getDataDirectory(), "system");
     }