Downgrade Android tests to JUnit 4

[yschimke]

Summary

• run the android-ech instrumentation suites with JUnit 4.13.2
• remove the Android JUnit 5 plugin, runner builder, and runtime dependencies
• convert Jupiter annotations and assumptions in both Android ECH test classes
• remove core-library desugaring that was only required by JUnit 5

Why

The Android JUnit 5 runner disables Jupiter tests below API 26. That prevents EncryptedClientHelloTest from running on the API 21 emulator even though the suite intentionally supports API 21 and skips only the TLS 1.3 fixture cases below API 29. Using AndroidX's native JUnit 4 runner keeps the instrumentation suite loadable across the full API matrix.

The JVM suites continue to use JUnit 5.

Validation

• ./gradlew :android-ech:compileDebugAndroidTestKotlin :android-ech:checkPublicApiOnly
• ./gradlew :android-ech:assembleDebugAndroidTest
• inspected debugAndroidTestRuntimeClasspath: JUnit 4.13.2 is present and the JUnit 5 platform/Android runner are absent