ignore new directory added in Android Studio 3.5 (#3232)

Author: shiftkey

Android.gitignore

@@ -56,6 +56,7 @@ captures/
 
 # External native build folder generated in Android Studio 2.2 and later
 .externalNativeBuild
+.cxx/
 
 # Google Services (e.g. APIs or Firebase)
 # google-services.json