Add documentation for *.sysprop

 * Gives access to the system properties store.  The system properties

 * store contains a list of string key-value pairs.

 *

 * <p>Use this class only for the system properties that are local. e.g., within

 * an app, a partition, or a module. For system properties used across the

 * boundaries, formally define them in <code>*.sysprop</code> files and use the

 * auto-generated methods. For more information, see <a href=

 * "https://source.android.com/devices/architecture/sysprops-apis">Implementing

 * System Properties as APIs</a>.</p>

 *

 * {@hide}

 */

@SystemApi