fix(braze): rename API key variables to App Identifier Key

[1abhishekpandey]

Summary

This PR updates the Braze integration to use Braze's official "App Identifier Key" terminology instead of the generic "API key" naming convention. This improves code clarity and aligns with Braze's documentation standards.

Changes

• Renamed internal constants and variables to use "App Identifier Key" terminology
  • Rename ANDROID_API_KEY to ANDROID_APP_IDENTIFIER_KEY
  • Rename API_KEY to DEFAULT_APP_IDENTIFIER_KEY
  • Rename USE_PLATFORM_SPECIFIC_API_KEYS to USE_PLATFORM_SPECIFIC_APP_IDENTIFIER_KEYS
• Updated log messages and error messages to reflect the correct terminology
• No functional changes - this is a pure refactoring for terminology consistency

🔒 Scanned for secrets using gitleaks 8.28.0

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• Version upgraded (project, README, gradle, podspec etc)
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added unit tests for the code
• I have made corresponding changes to the documentation

[1abhishekpandey]

Refer here: #52.