build(deps): bump Android Gradle Plugin to 9.3.0 on JDK 21 only

[tony19]

Summary

Adopts Android Gradle Plugin 9.3.0, but only on the JDK 21 build leg — superseding Dependabot's #438, which bumped AGP to 9.3.0 unconditionally and broke the JDK 17 CI build.

Why #438 fails

AGP 9.3.0's bundled Android Lint calls java.util.List.removeLast() — a method added to List in JDK 21 (SequencedCollection). Under JDK 17 that method doesn't exist, so ./gradlew lint crashes:

> Task :logback-android:lintAnalyzeJdk11Debug FAILED
  Unexpected failure during lint analysis (this is a bug in lint or one of the libraries it depends on)
  Message: 'java.lang.Object java.util.List.removeLast()'
  ... NoSuchMethodError ... BidirectionalTextDetector

Build (JDK 21) passed; only Build (JDK 17) failed, and only at the lint step (assemble, bytecode verification, and unit tests all pass under JDK 17 + AGP 9.3.0).

Approach

Keep JDK 17 coverage by making the AGP version a Gradle property instead of dropping the JDK 17 matrix leg:

• build.gradle — buildscript classpath reads com.android.tools.build:gradle:${agpVersion}
• gradle.properties — agpVersion defaults to 9.2.1 (the JDK-17-compatible version; also what the JDK-17 test_app publish step uses)
• .github/workflows/build.yml — the build matrix sets agp per leg (17 → 9.2.1, 21 → 9.3.0) and exports ORG_GRADLE_PROJECT_agpVersion, overriding the default only on the JDK 21 leg

Net effect: JDK 17 builds/lints/tests on AGP 9.2.1; JDK 21 exercises AGP 9.3.0. No JDK coverage is dropped.

Test plan

• Build (JDK 17) green on AGP 9.2.1
• Build (JDK 21) green on AGP 9.3.0 (lint included)
• Test App green (JDK 17, default AGP 9.2.1)

Closes #438.

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