SDK-1052: Replace the mobile attribution scripting define with a build settings checkbox

[nattb8]

What

• Adds two checkboxes (iOS, Android) to the existing AudienceMobileBuildSettings asset that read and write the AUDIENCE_MOBILE_ATTRIBUTION scripting define directly on Player Settings
• The checkboxes are not serialized fields, they reflect the real Player Settings state live every time the asset is opened, so a define set the old manual way already shows correctly and nothing resets on upgrade
• Removes three copies of the same define-check method, one per iOS post-processor, in favour of one shared MobileAttributionDefine helper
• Adds tests for the pure add/remove logic, without touching real Player Settings (that would trigger a mid-test script recompile)

Why

Turning on mobile attribution meant typing an exact define string into Player Settings by hand, once per platform, with no feedback if you got it wrong or forgot a platform. This replaces that with a checkbox on the same asset studios already configure for tracking usage description and SKAdNetwork IDs.

Test plan

• Manually verified in the Unity Editor: checkboxes render, reflect existing manually-set defines correctly, and toggling one only changes that platform (confirmed via a live before/after diff of ProjectSettings.asset, Android define cleared while iOS stayed untouched)
• Automated EditMode test run, blocked locally by a Unity batch-mode license activation issue unrelated to this change, tests should be run in CI or manually via Test Runner as part of review

Ticket: SDK-1052

🤖 Generated with Claude Code

[github-actions]

Audience SDK — Build Size

Platform	SDK Size	Change
Android	0.39 / 20.00 MB	+0.03 MB	✅
Windows	0.17 / 20.00 MB	+0.01 MB	✅
iOS	8.60 / 20.00 MB	+0.09 MB	✅
macOS	1.00 / 20.00 MB	+0.10 MB	✅

SDK Size = build minus empty app. Change = vs baseline. Fails if any platform exceeds its absolute size limit.