cordova-plugin-googlemaps: groundOverlay zIndex problem

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cordova information: (run $> cordova plugin list)

cordova-plugin-app-event 1.2.1 "Application Events"
cordova-plugin-chrome-apps-common 1.0.7 "Chrome Apps Common Utils"
cordova-plugin-chrome-apps-socket 1.2.3 "Chrome Apps Socket API"
cordova-plugin-chrome-apps-sockets-udp 1.3.0 "Chrome Apps Sockets UDP API"
cordova-plugin-chrome-apps-system-network 1.1.2 "Chrome System Network API"
cordova-plugin-compat 1.0.0 "Compat"
cordova-plugin-crosswalk-webview 2.3.0 "Crosswalk WebView Engine"
cordova-plugin-device 1.1.7 "Device"
cordova-plugin-file 4.3.3 "File"
cordova-plugin-file-transfer 1.6.3 "File Transfer"
cordova-plugin-firebase 0.1.24 "Google Firebase Plugin"
cordova-plugin-googlemaps 2.1.1 "cordova-plugin-googlemaps"
cordova-plugin-local-notification 0.8.4 "LocalNotification"
cordova-plugin-tts 0.2.3 "TTS"
cordova-plugin-whitelist 1.3.2 "Whitelist"

If you use @ionic-native/google-maps, please tell the package.json (only @ionic-native/core and @ionic-native/google-maps are fine mostly)

Current behavior: I created a groundOverlay png image in my map, but I don’t want the google map markers(below is screen capture highlight in red line) show on png image whenever set zIndex to 999… it’s still not working. Expected behavior: Just setting zIndex to make image higher than goggle map markers layer. Screen capture or video record:

Related code, data or error log (please format your code or data):

		map.addGroundOverlay({
			url: 'images/map.png',
			bounds: bounds,
			opacity: 1,
			zIndex: 999,
			clickable: true
		}, function(groundOverlay) {
			groundOverlay.on(plugin.google.maps.event.GROUND_OVERLAY_CLICK, function(event) {
				if (editable) {
					placeMarker(map, event.latLng);
				} else {
					here =
					{
						lat: event.lat,
						lng: event.lng
					}
				}
			});

		});