[Objective-C Style] Standardize `LensOverlayPanTrackerDelegate`

Adopt the general delegate style.

Bug: 400916185
Change-Id: I842abb8ec925e6850dd5037e7a8e15e1a646d51d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6330722
Commit-Queue: Ameur Hosni <ameurhosni@google.com>
Commit-Queue: Radu Nitescu <radunitescu@google.com>
Auto-Submit: Radu Nitescu <radunitescu@google.com>
Reviewed-by: Ameur Hosni <ameurhosni@google.com>
Cr-Commit-Position: refs/heads/main@{#1428852}