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/api/test-removed.txt b/api/test-removed.txt
index 9b5d3ab..36c8ce5 100644
--- a/api/test-removed.txt
+++ b/api/test-removed.txt
@@ -22,6 +22,8 @@
   public abstract class Context {
     method public deprecated android.content.Context createCredentialEncryptedStorageContext();
     method public deprecated android.content.Context createDeviceEncryptedStorageContext();
+    method public abstract android.content.SharedPreferences getSharedPreferences(java.io.File, int);
+    method public abstract java.io.File getSharedPreferencesPath(java.lang.String);
     method public deprecated boolean isCredentialEncryptedStorage();
     method public deprecated boolean isDeviceEncryptedStorage();
     method public deprecated boolean migrateDatabaseFrom(android.content.Context, java.lang.String);