This is how it looks like in my Android Studio 3.5 beta2:
You can see that I don’t hide obsolete packages and request package details. Now we see that the old NDK package is replaced by the new NDK (Side by side), which allows to keep both NDK r19 and r20.
The reason why they are making the change is that switching to the latest version of NDK is not always painless, and many developers prefer to stay with an older version, at least for some projects.
The bottom line is that it is OK to use the NDK that you have installed, but it will be replaced with the new package some time later when you upgrade the Android Studio.