fix: [SDK-4969] support AGP 9 Android builds

[fadi-george]

Description

One Line Summary

Adds Android Gradle Plugin 9 compatibility while preserving AGP 8 support for existing Capacitor hosts.

Details

Motivation

AGP 9 removes the library targetSdk setter, enables built-in Kotlin, hides legacy Android DSL implementations, and rejects proguard-android.txt. These changes prevented the Capacitor plugin from configuring and building in AGP 9 host applications. Fixes #41.

Scope

• Uses AGP 9 built-in Kotlin while retaining kotlin-android and explicit JVM targeting for AGP 8 hosts.
• Migrates the plugin to the public library DSL and removes library-only targetSdk configuration.
• Removes obsolete plugin ProGuard configuration and updates example applications to the supported optimized default.
• Upgrades the primary demo to AGP 9.1.1 and Gradle 9.3.1 so Android Appium E2E continuously exercises AGP 9.
• Keeps Capacitor 7 and the remaining Capacitor 8 fixtures on AGP 8 for compatibility coverage.
• Does not change the plugin's public JavaScript API.

Testing

Unit testing

No new unit tests were needed because the change is limited to Gradle configuration. Existing test suite passes: 179 tests with 100% statement coverage.

Manual testing

• Built the Capacitor 7 fixture with AGP 8.7.x, including Kotlin compilation.
• Built the Capacitor 8 fixture with AGP 8.13.x.
• Built the primary Capacitor 8 demo with AGP 9.1.1, Gradle 9.3.1, and JDK 21 without compatibility opt-out flags.
• Built the no-location Android fixture.
• Ran the primary demo's AGP 9 Android Appium suite: 32 passing, 2 skipped.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

[sherwinski]

It might be worth bumping one of the examples to use AGP 9, or create a new minimal one?