Android comes with an input command-line tool that can simulate miscellaneous input events.
To simulate a tap, use:
input tap x y
Run the input command remotely using adb shell:
adb shell input tap x y
Other options are:
shell@m0:/ $ input
input
usage: input ...
input text <string>
input keyevent <key code number or name>
input [touchscreen|touchpad|touchnavigation] tap <x> <y>
input [touchscreen|touchpad|touchnavigation] swipe <x1> <y1> <x2> <y2> [duration(ms)]
input trackball press
input trackball roll <dx> <dy>