Seems like there is an UI-Bug in the Android Studio (1.x, 2.x and 3.x).
For me the “Frames/Threads” Panel was completely hidden behind the toolbar, so I had to change the size from the “variable” panel by dragging its left border to reveal the “Frames/Threads”.
[I have to admit, that @Greg added this picture after reviewing my answer!]
At the bottom panel you should have “5: Debug”. Click on it and select “Debugger -> Threads”
You may need to find the “Threads” icon on the far right, or even click the “Restore Layout” button on the left to restore this window.