Is my Android device vulnerable to CVE-2019-2234 and How can I mitigate this before a firmware upgrade?
If I understood the problem correctly the vulnerability is located in the camera apps. Updating apps is the responsibility of the Google PlayStore or in case of Samsung there is also the Samsung "Galaxy Store" which is usually comes pre-installed on all Samsung devices.

Therefore in this case no firmware update is required. 

In both cases Google and Samsung have simply updated their camera apps. As the vulnerability was a responsible disclosure the updates that fix this vulnerability should be already available and will be automatically installed by the Google Play / Galaxy  store. 

Therefore Google and Samsung devices this vulnerability should be no longer an issue. The more important question is how many other camera apps of other manufacturer are also affected.

[Part 1] Regarding devices of other manufacturers there is no general way to be sure that their camera apps are affected or not. If the standard POC does not work and you want to make sure that the app is truly not affected you have to check every camera app individually if the vulnerability can be triggered in a similar way. To do so you have to decompile the app using the decompiler of your choice (Jadx, apktool, JEB, Ghidra, ...) and analyze the decompiled code.