Use return value instead of out param in GetServiceAndCallbackContext*

PrintBackendServiceManager::GetServiceAndCallbackContext*() methods
now return a ServiceAndCallbackContext struct, which contains both the
CallbackContext and mojom::PrintBackendService.

Bug: 1418830
Change-Id: I566440d80be7c76fb532b90c0aac4bbd8022cf40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5091987
Commit-Queue: chenguang shao <chenguangshao1@gmail.com>
Reviewed-by: Alan Screen <awscreen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1236116}