Fix net.android.init.gradle.kts for Gradle 9.x compatibility

[Copilot]

• Fix the net.android.init.gradle.kts resource file to handle nullable case using Kotlin's ?.let {} pattern
• Remove useless content-checking test from CopyResourceTests.cs
• Add AgpVersion and GradleVersion properties to AndroidGradleProject for version configuration
• Add proper end-to-end integration test BindLibraryWithMultipleGradleVersions that tests binding projects with multiple AGP/Gradle version combinations
• Use existing $(_AGPInitScriptPath) property as escape hatch for custom init scripts

Summary

This PR fixes the generated net.android.init.gradle.kts Kotlin DSL script to be compatible with Gradle 9.x by using Kotlin's null-safe ?.let {} pattern instead of explicitly checking containsKey and then accessing the potentially nullable value.

Escape Hatch Property

Users can now provide their own custom init.gradle.kts file by overriding the existing $(_AGPInitScriptPath) property:

<PropertyGroup>
  <_AGPInitScriptPath>$(MSBuildProjectDirectory)/My.init.gradle.kts</_AGPInitScriptPath>
</PropertyGroup>

When set to a path outside the intermediate output directory, the build will skip copying the built-in resource and use the user's file directly.

Integration Tests

The integration test verifies compatibility across multiple AGP/Gradle versions:

• AGP 8.5.0 with Gradle 8.7
• AGP 8.7.0 with Gradle 8.9
• AGP 8.8.0 with Gradle 8.10.2
• AGP 8.9.0 with Gradle 8.11.1
• AGP 9.0.0 with Gradle 9.1 (tests the Gradle 9.x stricter Kotlin type checking fix)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Generated net.android.init.gradle.kts incompatible with Gradle 9.x (Kotlin type safety)</issue_title>
<issue_description>## Summary

Microsoft.Android.Sdk generates Kotlin DSL scripts (net.android.init.gradle.kts) that are incompatible with Gradle 9.x due to stricter Kotlin type checking.

Error

When building a .NET Android project with Gradle 9.x, the following error occurs:

e: file:///.../obj/Debug/net10.0-android/gradle/net.android.init.gradle.kts:11:36: 
Argument type mismatch: actual type is 'String?', but 'Any' was expected.

FAILURE: Build failed with an exception.

* Where:
Initialization script '.../obj/Debug/net10.0-android/gradle/net.android.init.gradle.kts' line: 11

* What went wrong:
Script compilation error:

  Line 11: layout.buildDirectory.set(file(gradle.startParameter.projectProperties["netAndroidBuildDirOverride"]))
                                          ^ Argument type mismatch: actual type is 'String?', but 'Any' was expected.

Root Cause

The generated code in net.android.init.gradle.kts line 11:

layout.buildDirectory.set(file(gradle.startParameter.projectProperties["netAndroidBuildDirOverride"]))

• gradle.startParameter.projectProperties["..."] returns String? (nullable)
• Gradle 9.x's file() function requires Any (non-nullable)
• Gradle 9.x has stricter Kotlin type checking and rejects the nullable type

Suggested Fix

Handle the nullable case in the generated Kotlin script:

gradle.startParameter.projectProperties["netAndroidBuildDirOverride"]?.let { 
    layout.buildDirectory.set(file(it)) 
}

Environment

• Microsoft.Android.Sdk.Darwin: 36.0.0-preview.7.229
• Gradle: 9.0.0
• Target Framework: net10.0-android

Impact

This blocks .NET Android/MAUI projects from upgrading to Gradle 9.x and AGP 9.x.

Related

• Fix Android build failure by explicitly specifying build-tools 36.0.0 maui#33727 - Fix Android build failure by explicitly specifying build-tools 36.0.0</issue_description>

<agent_instructions>If we fix this,

• We need to detect both old & new gradle formats -- both need to work.

• We should write a test first that reproduces the issue.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Generated net.android.init.gradle.kts incompatible with Gradle 9.x (Kotlin type safety) #10737

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