[Printing] Fix collecting "PrintPreview.PrintAttemptOutcome" uma metric.

Set a proper default value for printer_manually_sected_. Why?
When printing without regular OS dialogs and printing previews, this
variable is never set, but when adding to "PrintPreview.PrintAttemptOutcome"
uma metric, the uninitialized value can cause trouble.

Bug: 414778579
Change-Id: I3ef307570446cc5b433e1ca83c96390be934e648
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6494574
Reviewed-by: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Dominik Bylica <bylica@google.com>
Cr-Commit-Position: refs/heads/main@{#1454101}