Widget Preview app missing from Android emulator
The Widget Preview app will only show up when running an older version of Android on an emulator (API 26 or lower).
If the Widget Preview isn't showing up on your emulator, you need to create a new Virtual Device with a lower API in Android Studio:
1) Tools>AVD Manager>Create Virtual Device...
2) Select the device you'd like to use, click "Next"
3) In the next window, select Nougat, API Level 25. You may need to download it if you've never used it before. Click "Next", then "Finish"
4) Run the app on the new virtual device, and search for the Widget Preview again
After the preview has been created, the emulator gives you the option of emailing it so you can save it locally and later to the drawables folder of your project.
Hope that solves it for you too!
Future readers, I solved this by using an older android version system image.