Clean up and rename the original template files

[quapka]

There seems to be files either misleadingly named or left over from the initial Flutter template (my assumption). Consider

$ ls $(fd example) -R
android/app/src/main/kotlin/com/example/:
meesign_client

android/app/src/main/kotlin/com/example/meesign_client:
MainActivity.kt

E.g. the linux/my_application.cc should be something like linux/meesign_client.cc, etc.
On one hand, this does not have a high priority, on the other hand, it would be nice to have this done to simplify navigation in project and understanding it.