feat(audience): resolve install referrer via Maven instead of bundling the AAR

[nattb8]

The Audience package bundled installreferrer-2.2.aar directly, and bundled AARs can't be de-duplicated by Gradle — so any game that already pulled the Google Play Install Referrer Library via another SDK hit class/version conflicts on the Android build. This swaps the bundled AAR for a Maven-resolved dependency so Gradle resolves a single highest version across all SDKs, fixing the conflict reported from customer.

Changes

• Remove the bundled installreferrer-2.2.aar (+ .meta).
• Add Editor/ImmutableAudienceDependencies.xml declaring com.android.installreferrer:installreferrer:2.2 for EDM4U's Android Resolver.
• Document a manual mainTemplate.gradle line so EDM4U is a recommended prerequisite, not a hard requirement.
• Add that same manual line to the examples/audience sample (it doesn't use EDM4U), which also serves as a worked example of the fallback.
• Retain the defensive R8 keep-rules in proguard-user.txt (the Maven artifact is the same AAR).

Notes

• play-services-ads-identifier (GAID) is intentionally still studio-provided, so games not collecting the advertising ID never declare AD_ID. Unchanged.
• The AUDIENCE_MOBILE_ATTRIBUTION define still gates the C# JNI code. The Maven dependency itself is no longer define-gated (EDM4U can't read scripting defines), so non-attribution builds carry a ~8 KB permissionless unused dependency — an accepted cosmetic cost over a Unity-version-fragile Gradle-injection post-processor.

⚠️ Breaking change

Studios using AUDIENCE_MOBILE_ATTRIBUTION now need either EDM4U (automatic) or the documented manual Gradle line. The package is alpha, so this is acceptable.

Testing

Verified with a batchmode Android build of examples/audience (Unity 2021.3.45f2, AUDIENCE_MOBILE_ATTRIBUTION set, manual-fallback gradle path — no EDM4U):

• Package imports and compiles cleanly with the AAR removed; the new Dependencies.xml/.meta import without error.
• Gradle resolved installreferrer:2.2 from Maven (confirmed in the manifest-merger report).
• The resulting App Bundle's dex contains the installreferrer classes (InstallReferrerClient, InstallReferrerClientImpl, ReferrerDetails, …).
• Build succeeded.

The EDM4U auto-resolution path (Dependencies.xml) was not exercised — this sample project has no EDM4U installed — so a reviewer with an EDM4U project should sanity-check that path.