IWeaver.aidl: document that throttling must be per-slot

     * Throttling must be used to limit the frequency of failed read attempts.

     * The value is only returned when throttling is not active, even if the

     * correct key is provided. If called when throttling is active, the time

     * until the next attempt can be made is returned.

     * until the next attempt can be made is returned. Throttling must be

     * applied on a per-slot basis so that a successful read from one slot does

     * not reset the throttling state of any other slot.

     *

     * Service status return:

     *