PrintPreview: Improve type checking when referring to Settings object.

Specifically using 'keyof Settings' instead of 'string' as the
TypeScript type whenever dereferencing the global Settings object, which
surfaces any typos, or references to missing keys. For example it was
revealed that 'recentDestinations' was erroneously not declared as a
key.

Bug: 331681689
Change-Id: I5868aadedd6179787889556872426e6a554340c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5416454


Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281919}