Add missing CODEOWNER (#7016)

`shared_preferences_android` was accidentally missing a code owner.

Author: stuartmorgan-g

CODEOWNERS

@@ -38,6 +38,7 @@ packages/in_app_purchase/in_app_purchase_android/**      @gmackall
 packages/local_auth/local_auth_android/**                @camsim99
 packages/path_provider/path_provider_android/**          @camsim99
 packages/quick_actions/quick_actions_android/**          @camsim99
+packages/shared_preferences/shared_preferences_android/**  @reidbaker
 packages/url_launcher/url_launcher_android/**            @gmackall
 packages/video_player/video_player_android/**            @camsim99