[build] Stabilize Android define constant ordering

[jonathanpeppers]

Summary

• append Android define constants at a fixed compiler stage after the .NET SDK implicit defines
• prevent equivalent project configurations from producing different compile dependency hashes
• add regression coverage for opposite target execution orders and disabled implicit framework defines

Testing

• PATH="/opt/homebrew/bin:$PATH" make all
• ./dotnet-local.sh test bin/TestDebug/net10.0/Xamarin.Android.Build.Tests.dll --filter 'Name~AndroidDefineConstantsAreOrderIndependent|Name~DesignTimeBuildHasAndroidDefines'

Fixes #12304

[Copilot]

Pull request overview

This PR addresses incremental build churn caused by non-deterministic ordering of Android define constants by appending Android defines at a consistent point in the compile pipeline (after .NET SDK implicit defines). This helps prevent equivalent project configurations from producing different compile dependency hashes and triggering unnecessary recompiles and downstream APK repackaging.

Changes:

• Move the @(AndroidDefineConstants) append into a dedicated _AddAndroidDefineConstants target scheduled before compilation.
• Add a regression test that exercises opposite target execution orders to ensure DefineConstants ordering stays stable.
• Expand design-time build coverage to validate Android defines with implicit framework defines enabled/disabled.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets	Adds a pre-compile target to append Android define constants in a stable stage after SDK implicit defines.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs	Adds regression coverage to ensure define constant ordering is independent of target execution order.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest2.cs	Extends design-time build test matrix to include disabled implicit framework defines.

[jonathanpeppers]

@dalexsoto review

[dalexsoto]

The Android define append now runs at a stable compiler stage after implicit defines, with regression coverage proving target-order independence and design-time behavior for both implicit-define modes.