Bug 1600623. Add telemetry probes for print dialog/preview opens/cancels and print target type. r=bobowen,mbalfanz a=jcristau

The probes collect counts for:

 - print preview open, and exit without print
 - print dialog opened from print preview, and cancelled
 - print dialog opened without print preview, and cancelled
 - silent prints
 - print target
   - PDF file
   - XPS file
   - other (probably print to physical printer, but we can never be sure)

There is some overlap with the existing PRINT_* probes, but I think we should
keep those in place temporarily until we confirm that the new probes produce
numbers that are consistent with the old probes.

This patch only adds 'print target' probes for Windows and macOS.

I use nsDeviceContextSpec*::Init() to collect the 'print target' telemetry
because the way we initialize settings from prefs (and the way macOS works in
particular) make it difficult to reliably determine the target type earlier in
the print process for all possible entry points into the printing code.

Differential Revision: https://phabricator.services.mozilla.com/D78033