Fix documentation for feature flag reader

 * previous example, do the following:

 *

 * {@code

 *  $ adb shell device_config put systemui flag_foo_bar_baz true

 *  $ adb shell setprop persist.systemui.flag_foo_bar_baz 1

 * }

 *

 * Note that all storage keys begin with "flag_", even if their associated resId does not.

 *

 * Calls to this class should probably be wrapped by {@link FeatureFlags}.

 * Calls to this class should probably be wrapped by a method in {@link FeatureFlags}.

 */

@SysUISingleton

public class FeatureFlagReader {