ozone: Refactor imposter_checker_evdev

Moves StandardizedPhys to an unnamed namespace and makes a few small
refactors to imposter_checker_evdev. Addresses comments: [1]

[1] https://chromium-review.googlesource.com/c/chromium/src/+/5035438/comments/1a0d978f_68cca2c4?tab=comments

Bug: b:332778075
Test: Ran events_unittest
Change-Id: I7402d51c88f10a9f9457517aaa50e9ce0abf230f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5422288


Reviewed-by: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Kenneth Albanowski <kenalba@google.com>
Commit-Queue: William Mahon <wmahon@google.com>
Cr-Commit-Position: refs/heads/main@{#1283400}