[image_picker_ios] Update image picker UI test query for iOS 18

[jmagman]

The image picker UI changed from iOS 17 -> iOS 18 (beta), and there's an additional scrollview that wasn't present before.
Instead of selecting the first image in the first scroll view, instead select the image with the accessibility label that starts with "Photo":

      ↪︎Find: First Match
        Output: {
          Image, 0x10134f070, {{-0.0, 339.7}, {142.2, 142.3}}, label: 'Photo, March 30, 2018, 12:14 PM'
        }

This works on previous versions of iOS as well.

Fixes flutter/flutter#150220, found in Xcode 16 beta.

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[jmagman]

Passes on Xcode 16 beta 4 #7226

** TEST SUCCEEDED **

Testing started
Successfully ran iOS xctest for packages/image_picker/image_picker_ios/example

[packages/image_picker/image_picker_ios completed in 1m 54s]