Add OBB flags to support overlays

* Add flags field in OBB footer to support overlays.

* Remove unused 'crypto' and 'filesystem' fields in obbtool (could
  later be supported in the "flags" field of the OBB footer).

* Add notes to document OBB classes before shipping.

Change-Id: I386b43c32c5edef55210acb5d3322639c08010ba
diff --git a/core/java/android/os/storage/StorageManager.java b/core/java/android/os/storage/StorageManager.java
index cb1794f..df0b69c 100644
--- a/core/java/android/os/storage/StorageManager.java
+++ b/core/java/android/os/storage/StorageManager.java
@@ -304,6 +304,8 @@
      * file matches a package ID that is owned by the calling program's UID.
      * That is, shared UID applications can obtain access to any other
      * application's OBB that shares its UID.
+     * <p>
+     * STOPSHIP document more; discuss lack of guarantees of security
      * 
      * @param filename the path to the OBB file
      * @param key decryption key
@@ -328,6 +330,8 @@
      * file matches a package ID that is owned by the calling program's UID.
      * That is, shared UID applications can obtain access to any other
      * application's OBB that shares its UID.
+     * <p>
+     * STOPSHIP document more; discuss lack of guarantees of security
      * 
      * @param filename path to the OBB file
      * @param force whether to kill any programs using this in order to unmount