Obviously Android Device Monitor freezes right after launch when running with Java versions > 1.8.0_151.

To keep the most recent Java version installed on your machine, but run the Android Device Monitor with an older version, just change the start parameters for the monitor app package as follows:

1. Go to <android-sdk-homedir>/tools/lib/monitor-x86_64/
2. Right click on monitor.app –>  „Show Package Contents“
3. edit Info.plist in a text editor
4. Uncomment one of the <string>-vm</string> prepared templates and let it point to an older JDK installation on your machine, e.g.:

    <string>-vm</string><string>/Library/Java/JavaVirtualMachines/jdk1.8.0_65.jdk/Contents/Home/bin</string>

5. Save the file and start Android Device Monitor, should work now

(thanks also to https://stackoverflow.com/a/49389289)