You should still be able to target `net6.0-android31`, as I hardcoded
the version to what is released on NuGet.org:

https://www.nuget.org/packages/Microsoft.Android.Ref.31/31.0.101-preview.11.117

With these changes `net6.0-android31` resolves from:

    ~\.nuget\packages\microsoft.android.ref.31\31.0.101-preview.11.117\ref\net6.0\Mono.Android.dll

But with `net6.0-android` (defaults to 32 now), you get:

    dotnet\packs\Microsoft.Android.Ref.32\*\ref\net6.0\Mono.Android.dll

I also removed `%(DefaultRuntimeFrameworkVersion)` from
`@(KnownFrameworkReference)` as it is not needed.

Any app project build was hitting the warning:

    Xamarin.Android.Aapt2.targets(157,3): warning APT2000: aapt2 W 01-05 23:58:06  8352 43857 LoadedArsc.cpp:657] Unknown chunk type '200'.

And so any MSBuild test that asserts no warnings would fail.

I think we need to get a new version of `aapt2` to fix this.

Apps with the `AndroidManifest.xml`:

    <uses-sdk android:minSdkVersion="32" android:targetSdkVersion="32" />

Passes `--min-api 32` to d8/r8.

This emits the warning:

    D8 : warning : An API level of 32 is not supported by this compiler. Please use an API level of 31 or earlier

Let's remove this test case for now.

Otherwise, builds using `AndroidDexTool=dx` fail with:

    COMPILETODALVIK : error : Unable to access jarfile C:\Users\jopepper\android-toolchain\sdk\build-tools\32.0.0\\lib\dx.jar

It looks like we have existing logic falling back to older `build-tools` when you install them side-by-side.

* The default `$(TargetPlatformVersion)` is 32.0
* Keep the `Microsoft.Android.Ref.31` packs in our workload, hardcoded to the previous release on NuGet.org.
* Import `Microsoft.Android.Sdk.SupportedPlatforms.targets` *before* `Microsoft.Android.Sdk.BundledVersions.targets`

We get an XA1008 when targetSdkVersion="32" and $(TargetPlatformVersion)=31.0 otherwise. This causes various MSBuild tests to fail that assert no warnings.

…tFrameworkVersion>

We have to declare this after importing Configuration.props.

* Renamed `$(AndroidTargetDotNetApiLevel)` -> `$(AndroidDefaultTargetDotnetApiLevel)`
* Put this value in `XABuildConfig`, so we can use it in tests
* Updated several places to use `XABuildConfig` instead of 31 hardcoded inline