[cleanup] PrintBackendService initialization

PrintBackendServiceTestImpl::Init() does not invoke any initialization
in the base class.  This could result in unexpected differences for
developers that need to make changes in PrintBackendServiceImpl::Init().

Improve on this by ensuring the test class makes a call to the base
class for initialization.  Note that there is some initialization which
is for production-only and is not safe to do from within test
frameworks.  Document this condition and introduce a common
initialization method that test and production can share, so that any
future modifications to initialization sequence can be applied in the
correct place.

LOW_COVERAGE_REASON=Covered primarily by browser tests.

Change-Id: Ia7b34886b0b93cd9b17ea4bc977657c41dd3bf38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3905609
Commit-Queue: Alan Screen <awscreen@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1048864}