Fix ps check of child pids on BSD-based MacOS. Use pgrep instead.

[stuartmls]

Issue:

Unregistering a service results in the following error:

ps: illegal option -- -
usage: ps [-AaCcEefhjlMmrSTvwXx] [-O fmt | -o fmt] [-G gid[,gid...]]
          [-g grp[,grp...]] [-u [uid,uid...]]
          [-p pid[,pid...]] [-t tty[,tty...]] [-U user[,user...]]
       ps [-L]

Call-stack:

_collect_process_tree (lib/python3.12/site-packages/mobly/utils.py:271) _kill_process_tree (lib/python3.12/site-packages/mobly/utils.py:316) stop_standing_subprocess (lib/python3.12/site-packages/mobly/utils.py:555) _stop_server (lib/python3.12/site-packages/mobly/controllers/android_device_lib/snippet_client_v2.py:699) stop (python3.12/site-packages/mobly/controllers/android_device_lib/snippet_client_v2.py:660) remove_snippet_client (lib/python3.12/site-packages/mobly/controllers/android_device_lib/services/snippet_management_service.py:110) unload_snippet (lib/python3.12/site-packages/mobly/controllers/android_device.py:963) stop (lib/python3.12/site-packages/snippet_uiautomator/uiautomator.py:227) unregister (lib/python3.12/site-packages/mobly/controllers/android_device_lib/service_manager.py:109)

Sample code:

from mobly.controllers import android_device
from snippet_uiautomator import uiautomator

ad = android_device.AndroidDevice(sn)
ui = uiautomator.UiAutomatorService
ad.services.register(uiautomator.ANDROID_SERVICE_NAME, ui)
...    
ad.services.unregister(uiautomator.ANDROID_SERVICE_NAME)

---

This change is 

Co-authored-by: [email protected]

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[mhaoli]

Thanks for noticing this bug and creating the fix. Could you help add unit tests for it?

[mhaoli]

Closing this as I already fixed this error in #920