When trying to execute ./adb shell or adb shell being connected to Android Things via serial console I get ./adb: not found and adb: not found respectively.
I am connected via serial port to my raspberry 3
Once you got serial console connected you're already "shell"-ed in, meaning there is no need to execute adb for (further) connection.
Moreover, the adb binary is missing under /system/bin of Android Things (at least by now). This is the reason why you get adb: not found.
Summing up, when connected to serial console you should execute commands without adb or adb shell.