Restart adbd as shell user on a non-rooted device without rebooting after the "setprop" operation
In the specific context of setting the adb tcp/ip port, the adb tcpip 5555 command does restart the adbd service.

As one can see the PID of adbd is changed after the adb tcpip command :

$ adb shell ps | egrep -w "adbd|PID"
USER     PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
shell     3675  1     4720   300   ffffffff 00000000 S /sbin/adbd
$ adb tcpip 5555
restarting in TCP mode port: 5555
$ adb shell ps | egrep -w "adbd|PID"
USER     PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
shell     3800  1     4720   300   ffffffff 00000000 S /sbin/adbd