Merge "Docs: be explicit about oldState contract for backup helpers"

     * new state after performing the backup operation.

     * <p class="note">

     * <strong>Note:</strong> The helper should not close or seek either the {@code oldState} or

     * the {@code newState} file descriptors.</p>

     * the {@code newState} file descriptors.  It is essential that when reading the helper's

     * saved state from the {@code oldState} file, no extra content is consumed beyond

     * what was stored by this helper.  If more old state data is read, even accidentally,

     * it will make it impossible for additional helpers that may be invoked after this one

     * to properly reconstruct their prior state.</p>

     *

     * @param oldState An open, read-only {@link android.os.ParcelFileDescriptor} pointing to the

     *            last backup state provided by the application. May be