feat(GooglePlacesSwiftXCFrameworkDemos): read API key from Secrets.xcconfig [5/8]

[iHackSubhodip]

What

• Deletes tracked SDKDemoAPIKey.swift; reads PLACES_API_KEY from the bundle via
  Bundle.main.object(forInfoDictionaryKey:), fail-closed.
• Adds tracked GooglePlacesSwiftXCFrameworkDemos.xcconfig wrapper (#include? "Secrets.xcconfig").
• Info.plist maps PLACES_API_KEY → $(PLACES_API_KEY).
• project.pbxproj: wires the wrapper as a project-level base configuration.

Why

• Removes the key from a tracked source file; CocoaPods-safe (project-level base
  config survives pod install).
• Unifies onto the shared PLACES_API_KEY + Secrets.xcconfig mechanism.

When

• Series [5/8]; second and last CocoaPods app. Unlike feat(GooglePlacesSwiftDemos): API key via Secrets.xcconfig + fix Pods [4/8] #31, no Podfile change —
  this target was already in the Podfile.

Testing

• Open GooglePlaces-Swift.xcworkspace, add GooglePlacesSwiftXCFrameworkDemos/Secrets.xcconfig
  with PLACES_API_KEY = … → launches; remove it → builds, fail-closed.

Simulator.Screen.Recording.-.iPhone.17.-.2026-07-29.at.17.41.17.mov