Settings: Simplify settings-prefs lastPrefValues_ property.

Specifically
 - Stop declaring it as a Polymer property, since it is not necessary.
   Just a class member is sufficient.
 - Use a Map instead of a plain object. Since the code needs to check
   if keys exist, or clear all keys, a Map is more appropriate.

Bug: 331681689
Change-Id: I104e4ef45a9a22143dfdc94033ad14a66d9d34d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5441178
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1284788}