Agents with dalvik request to server contain some characters after Build. for example:
Dalvik/1.6.0 (Linux; U; Android 4.4.4; C6902 Build/14.4.A.0.157)
Dalvik/2.1.0 (Linux; U; Android 8.0.0; SM-G930F Build/R16NW)
Question: Is it unique for any android device/emulator?
Those characters represent the build "number" which is the current build version running on the device. If I understand correctly your question then no, it cannot be use to identify a unique device.
You can check the Android source website and verify the current build version for devices (like the Pixel phones) running Android 8.1. Those alphanumeric characters are used to identify the Android version built for a specific device, but you can't identify a unique device based on it.