[iOS Demo App] Fix retain cycles in bottom sheet demo

[anandrajeswaran]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

We have a few retain cycles (noted by #2106), one where a strong reference to self is sent to a block stored by a strong var of self, the other where a strong reference to a view controller is sent to a block stored by a view in that view controller's hierarchy. Fix both using weak references in these blocks.

Binary change

N/A - No product change

Verification

Sanity in the test app by temporarily adding a deinit to the view controller and a breakpoint confirmed that it is now called when the view controller is dismissed.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

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