[TrimmableTypeMap] Generate the typemap before the NativeAOT inner build so the IDE Compile→Sign flow works (#11789)

### Summary

The NativeAOT trimmable typemap (the generated Java ↔ .NET type map) is consumed by the **inner per-RID ILC build** — via `_AddTrimmableTypeMapAssembliesToIlc` → `_ReadGeneratedTrimmableTypeMapAssemblies` — which reads the generated assembly list at `obj/.../typemap/typemap-assemblies.txt`. The inner build cannot *generate* that list: `_GenerateTrimmableTypeMap` is gated to the **outer** build (`'$(_OuterIntermediateOutputPath)' == ''`).

`_GenerateTrimmableTypeMap`'s only trigger is `AfterTargets="CoreCompile"`. That hook does not fire when compilation is already up-to-date — which is exactly what happens in the IDE's split build, where Visual Studio (`BuildingInsideVisualStudio=true`) runs **`Compile`** and **`SignAndroidPackage`** as two separate MSBuild invocations. During the `SignAndroidPackage` call the C# is already compiled, so `CoreCompile` is skipped, the typemap is never generated, and the inner ILC build fails with:

```
Trimmable typemap assembly list '.../typemap/typemap-assemblies.txt' was not found
```

A regular command-line `Build` works because all targets run in one continuous invocation; only the IDE Compile→Sign sequence hits this. Reproduced by the `DesignTimeBuildSignAndroidPackage(NativeAOT)` test.

Fixes #11775.

### Fix

Add a target that forces the typemap to be generated in the **outer** build before `_ResolveAssemblies` spawns the inner per-RID ILC build:

```xml
<Target Name="_EnsureTrimmableTypeMapGeneratedForNativeAotInnerBuild"
    Condition=" '$(RuntimeIdentifier)' == '' "
    BeforeTargets="_ResolveAssemblies"
    DependsOnTargets="_GenerateTrimmableTypeMap" />
```

Instead of relying on the fragile `AfterTargets="CoreCompile"` hook, generation is anchored to `_ResolveAssemblies`, which always runs before the inner build and runs *after* compilation (it consumes `@(IntermediateAssembly)`), so the generator still observes the compiled app assembly. This mirrors the CoreCLR `_AddTrimmableTypeMapToLinker` target, which forces generation before the (outer) ILLink.

### Risk

Low and well-scoped: the new target is conditioned to the outer build (`RuntimeIdentifier == ''`) and lives in the NativeAOT trimmable targets file; it only ensures an already-required step has run. No effect on CoreCLR, managed, or MonoVM paths.

### Testing

Verified locally: `DesignTimeBuildSignAndroidPackage(NativeAOT)` now passes (was failing); `DesignTimeBuildSignAndroidPackage(CoreCLR)` continues to pass.

Sliced out of #11617 for focused review.

Author: simonrozsival

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.NativeAOT.targets

@@ -41,6 +41,31 @@
     </ItemGroup>
   </Target>
 
+  <!--
+    Force the typemap to be generated in the OUTER build (the build that is not an inner
+    per-RID build, i.e. '$(_OuterIntermediateOutputPath)' == '') before _ResolveAssemblies
+    spawns the inner per-RID ILC build. The inner build consumes the typemap assembly list
+    (via _AddTrimmableTypeMapAssembliesToIlc -> _ReadGeneratedTrimmableTypeMapAssemblies)
+    but cannot generate it: _GenerateTrimmableTypeMap is gated to the outer build (it requires
+    '$(_OuterIntermediateOutputPath)' == ''). _GenerateTrimmableTypeMap's own AfterTargets="CoreCompile"
+    trigger does not fire when compilation is up-to-date - e.g. the IDE's separate "Compile" then
+    "SignAndroidPackage" invocations (BuildingInsideVisualStudio=true) - leaving the file ungenerated
+    and the inner build failing with "Trimmable typemap assembly list ... was not found".
+    This mirrors the CoreCLR _AddTrimmableTypeMapToLinker target, which forces generation before the
+    (outer) ILLink. _ResolveAssemblies runs after compilation (it consumes @(IntermediateAssembly)), so
+    the generator still observes the compiled app assembly.
+
+    The condition matches on '$(_OuterIntermediateOutputPath)' == '' (the same discriminator
+    _GenerateTrimmableTypeMap uses) rather than 'RuntimeIdentifier == ''' so that single-RID outer
+    builds (e.g. -p:RuntimeIdentifier=android-arm64) are covered too; the inner build sets
+    _OuterIntermediateOutputPath via _ResolveAssemblies AdditionalProperties, so this target is
+    correctly skipped there.
+  -->
+  <Target Name="_EnsureTrimmableTypeMapGeneratedForNativeAotInnerBuild"
+      Condition=" '$(_OuterIntermediateOutputPath)' == '' "
+      BeforeTargets="_ResolveAssemblies"
+      DependsOnTargets="_GenerateTrimmableTypeMap" />
+
   <Target Name="_CollectTrimmableNativeAotDgmlFiles"
       Condition=" '$(PublishTrimmed)' == 'true' and '$(_ProguardProjectConfiguration)' != '' ">
     <ItemGroup>