post_process: implement RunActivities RPC for the service proto update

This is the dependent CL for the post process service proto update CL: https://crrev.com/c/5561545

CQ check failed for the proto update CL due to the error: "missing method RunActivities"
https://logs.chromium.org/logs/chromeos/buildbucket/cr-buildbucket/8747080019646313985/+/u/update_sdk/dev-util_post-process_log

Thus, a RunActivities would need to be added to the
PostTestServiceServer struct first.

Also refactored some of the related code for simplicity and readability.

BUG=b:340686431
TEST=Manual testing with a leased DUT: http://gpaste/5910166155558912
Change-Id: I62c0d87dccf077010e56360e7d93b19e0041d926
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/5569282
Tested-by: Zhihui Xie <[email protected]>
Commit-Queue: Zhihui Xie <[email protected]>
Reviewed-by: Azizur Rahman <[email protected]>
Reviewed-by: Jason Kusuma <[email protected]>
6 files changed