[A11y metrics] Set ChromeVox usage metric to never expire

This CL updates one of the accessibility metrics to never expire. This
metric tracks the overall usage of ChromeVox and is considered a
guardrail metric internal to the accessibility team and should never
expire. This aligns the expiration with that of the equivalent metrics
on other platforms:

  Accessibility.Android.ScreenReader{AccessibilityEveryReport}
  Accessibility.Mac.ScreenReader{AccessibilityEveryReport}
  Accessibility.WinJAWS{AccessibilityEveryReport}
  Accessibility.WinNVDA{AccessibilityEveryReport}
  Accessibility.WinScreenReader2{AccessibilityEveryReport}
  Accessibility.WinSupernova{AccessibilityEveryReport}
  Accessibility.WinZoomText{AccessibilityEveryReport}

AX-Relnotes: N/A
Bug: 331857972
Change-Id: Ic8712d9e476748f2d5c22c7ceb1a0e4fe6f4c27d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420236


Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283139}