Notes to self

Save a lot of time if you're having problems with NDK, build and gradle and the like

Try different NDK versions.
Update the Gradle plugin. This is actually more tricky than one might think. Proceed as follows: Copy the gradle from an existing project. for example: cp /home/demo/ncnn-android-nanodet/gradle/wrapper/gradle-wrapper.properties . Then restart. Android studio will prompt you to update the gradle plugin. You may have to select the version in File -> Settings -> build, execution and development -> gradle. There, you may have to select the gradle-wrapper.properties. It should recognize it automatically though.
change the cmake version (I think this finally solved it. For old projects, you have to try different versions. I downgraded to /home/demo/Android/Sdk/cmake/3.10.2.4988404 )
path to cmake is in local.properties. Note however, you shall not add the ndk.dir to the local.properties -> deprecated
Another tip, I'm not quite sure if this is it (use as last resort) , but when you have trouble with ndk version: File -> project structure -> SDK Location -> click on download Android NDK
go to File -> Project Structure, change the Gradle plugin version to the latest. List of the Exceptions I have encountered:
Invalid revision number
no signature of method: build.android is applicable for arguments types ...
ndk not configured