[Xamarin.Android.Build.Tasks] improve incremental design-time builds

[jonathanpeppers]

I was testing .NET MAUI projects inside Visual Studio, and noticed on
solution load two sets of builds run for each $(TargetFramework):

1. CollectPackageReferences;CollectFrameworkReferences;CollectUpToDateCheckBuiltDesignTime;CollectPackageDownloads;GenerateSupportedTargetFrameworkAlias;CollectAnalyzersDesignTime;CollectUpToDateCheckInputDesignTime;CollectUpToDateCheckOutputDesignTime;CollectSuggestedWorkloads;CollectCentralPackageVersions;CompileDesignTime;CollectResolvedCompilationReferencesDesignTime
2. ResolveFrameworkReferencesDesignTime;ResolveProjectReferencesDesignTime2;CollectResolvedSDKReferencesDesignTime;ResolveComReferencesDesignTime;ResolveAssemblyReferencesDesignTime;ResolvePackageDependenciesDesignTime;CollectSuggestedWorkloads

1. net6.0-maccatalyst 0.344s
2. net6.0-ios 0.353s
3. net6.0-android 1.466s
4. net6.0-maccatalyst 0.063s
5. net6.0-ios 0.059s
6. net6.0-android 1.424s

Android is the worst in both cases, due to:

GenerateResourceDesigner 737ms
GenerateResourceDesigner 766ms

I'm not sure how I got in this state, but the
_ManagedUpdateAndroidResgen MSBuild target never skips for me:

Building target "_ManagedUpdateAndroidResgen" completely.
Input file "C:\Program Files\dotnet\sdk-manifests\6.0.100\microsoft.net.sdk.maui\WorkloadManifest.targets" is newer than output file "obj\Debug\net6.0-android\designtime\Resource.designer.cs".

Reviewing the <GenerateResourceDesigner/> MSBuild task, there is a
case it doesn't update the timestamp on the Resource.designer.cs
output file. I added a <Touch/> call to solve this issue:

<Touch Condition="Exists ('$(_AndroidManagedResourceDesignerFile)')" Files="$(_AndroidManagedResourceDesignerFile)" />

This is a nice solution, as it is localized to the MSBuild target where the
Inputs and Outputs are.

While I was at it, I switched from using a new StreamWriter() to
MemoryStreamPool. MemoryStreamPool usage saw a modest improvement:

Before:
GenerateResourceDesigner 491ms
After:
GenerateResourceDesigner 480ms

Overall these changes should improve all design-time builds by ~11ms,
and some incremental design-time builds will properly skip: saving up
to ~766ms.

[jonpryor]

Alternate solution idea: instead of updating <GenerateResourceDesigner/> to "touch" the file, thus implicitly requiring that that the task "know about" it's calling target, why don't we instead update the _ManagedUpdateAndroidResgen target?

<Touch Files="$(_AndroidManagedResourceDesignerFile)" />

This keeps the logic within the target.

[jonathanpeppers]

I switched to using <Touch/>:

I looked through the other code that uses CopyIfChanged. It appears this issue is only in the _ManagedUpdateAndroidResgen target.

[dellis1972]

This looks good. the commit message needs updating to reflect the new changed (i.e using Touch).

[jonathanpeppers]

I updated the PR description, we can copy paste that when we merge.