[Mono.Android] Default to SocketsHttpHandler in .NET 12 - #13013

Use the shared System.Net.Http.SocketsHttpHandler implementation by default in .NET 12 instead of the Android-specific HTTP handler. This reduces reliance on a separate implementation with differing exception and cancellation behavior.

• Default UseNativeHttpHandler to false, preserving explicit overrides.
• Mark AndroidMessageHandler obsolete with a warning recommending SocketsHttpHandler; applications can still opt into the native handler.
• Document the new default and deprecation, and suppress obsolete warnings in tests that intentionally exercise the legacy handler. No new unit test is included, as requested during review.
• Update the two MAUI CoreCLR .apkdesc baselines using measured Release CI output, accepting the size increase without relaxing regression thresholds.

Accepted APK size increase

Switching to the sockets handler increases the measured MAUI CoreCLR APK size by 897,024 bytes (876 KiB) in both configurations. This is an intentional, accepted tradeoff for using the shared HTTP implementation.

The updated references exactly match the complete descriptors emitted by Release CI build 1624599, including the small accompanying native-library/DEX changes. Both simple-app CoreCLR size checks passed, so their references are unchanged. These are CI measurements, not locally generated measurements; local SDK preparation remains blocked as described below.

HTTP/3 is already disabled by default, and HTTP tracing, metrics, and EventSource support are already disabled in optimized builds as appropriate. The runtime switch audit did not identify an additional Android networking trimming switch to safely remove this increase. Disabling managed NTLM through _UseManagedNtlm=false does not gate Android's authentication implementation.

Related issues and PRs

These links provide implementation history and compatibility context; this PR does not claim to close them.

• #5646 originally set UseNativeHttpHandler=true; this PR reverses that default for .NET 12.
• #12934 documents remaining upload-exception differences between the native and sockets handlers.
• #11682 and #11683 are earlier native-handler exception/cancellation alignment fixes.
• dotnet/runtime#47083 introduced platform-specific handler selection and the feature switch.
• dotnet/runtime#55384 added native underlying-handler support to HttpClientHandler, alongside SocketsHttpHandler.
• dotnet/runtime#109532 tracks an Android sockets-handler client-certificate diagnostic limitation; retaining the explicit native-handler opt-in provides compatibility for affected applications.

Validation

• Passed: dotnet msbuild src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets -nologo -p:AndroidApplication=true -p:AndroidMinimumSupportedApiLevel=24.0 -p:Configuration=<Debug|Release> -p:_AndroidRuntime=<CoreCLR|NativeAOT> [-p:UseNativeHttpHandler=<true|false>] -getProperty:UseNativeHttpHandler across all 12 configuration/runtime/unset-or-explicit-value combinations. Unset evaluates to false; explicit values are preserved.
• Passed: git diff --check.
• Passed: parsed both updated references as JSON and compared all 736 entries and package metadata in each against the complete descriptors emitted by Release CI build 1624599; both match exactly.
• CI build 1624599: BuildReleaseArm64(False,CoreCLR,False) and BuildReleaseArm64(False,CoreCLR,True) passed. The two MAUI variants built successfully but failed against the previous size references; those references are now updated. Validation of the new baseline commit is pending CI.
• Blocked: make prepare && make all failed during preparation/restore with NU1102: Microsoft.NETCore.App.Ref version 10.0.13 is unavailable from the configured feeds. make all did not run.
• Blocked: make prepare CONFIGURATION=Release && make all CONFIGURATION=Release failed during preparation/restore for the same missing reference-pack version. The Release SDK build did not run.
• Blocked: dotnet build src/Mono.Android/Mono.Android.csproj -v quiet failed during dependency restore for the same missing reference-pack version.
• Local full-build and on-device tests were not run because SDK preparation/build is blocked.

Pull Request
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• Useful description of why the change is necessary.
• Links to issues fixed
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@wfurt fyi