Demos: Git ignore Android Studio .cxx folder

Author: greenrobot-team

objectbox/example/flutter/objectbox_demo/.gitignore

@@ -38,6 +38,7 @@ app.*.symbols
 app.*.map.json
 
 # Android Studio will place build artifacts here
+/android/app/.cxx
 /android/app/debug
 /android/app/profile
 /android/app/release

objectbox/example/flutter/objectbox_demo_relations/.gitignore

@@ -38,6 +38,7 @@ app.*.symbols
 app.*.map.json
 
 # Android Studio will place build artifacts here
+/android/app/.cxx
 /android/app/debug
 /android/app/profile
 /android/app/release

objectbox/example/flutter/objectbox_demo_sync/.gitignore

@@ -38,6 +38,7 @@ app.*.symbols
 app.*.map.json
 
 # Android Studio will place build artifacts here
+/android/app/.cxx
 /android/app/debug
 /android/app/profile
 /android/app/release