Reduce platform-like buildflags in sandbox code

Sandbox code is difficult to follow if there are too many
conditional defines and it is ok for chromium to support
a sandbox type that it doesn't use in some configurations.

This CL makes two sandbox types (kPrintBackend and kScreenAI)
always be defined and supported on the platforms where they
are used (they are always used in official Chrome builds on
these platforms).

Both ENABLE_SCREEN_AI_SERVICE and ENABLE_OOP_PRINTING are
always enabled on linux, cros, mac and win.

Additionally some buildflags were tested where they were always
true, so those ifdefs are removed (e.g. in
utility_process_sandbox_browsertest.cc).

Bug: 41494527
Change-Id: Ief90cf997da5677b54e064abd7a45f6eaf3cfebe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6015430
Commit-Queue: Alex Gough <ajgo@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Mark Rowe <markrowe@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1386974}