[ios] Update shouldUseColorLensAndVoiceIcons()

Follow-up of:
https://chromium-review.googlesource.com/c/chromium/src/+/6040004

In previous change shouldUseColorLensAndVoiceIcons() was updated only
if ios_enable_widgets_for_mim was not enabled.

Moreover, since the logic is the same, this CL refactors the code to
have only one implementation of that method.

Bug: 361121027
Change-Id: I3344b6c0e700587d4bdd5e058c08fb43ba0fc671
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6141657
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Auto-Submit: Federica Germinario <fedegermi@google.com>
Cr-Commit-Position: refs/heads/main@{#1402375}