[One .NET] implement $(SupportedPlatformOSVersion) for minSdkVersion

[jonathanpeppers]

Context: #6107 (comment)

In .NET 6, we are wanting AndroidManifest.xml in project templates
to no longer contain:

<uses-sdk android:minSdkVersion="21" android:targetSdkVersion="30" />

This is because we can define these values such as:

<TargetFramework>net6.0-android30</TargetFramework>
<SupportedPlatformOSVersion>21.0</SupportedPlatformOSVersion>

There is not really a reason to put this information in two places.
$(SupportedPlatformOSVersion) is also required for enabling C#
compiler warnings such as:

MainActivity.cs(28,4): warning CA1416: This call site is reachable on: 'Android' 21.0 and later. 'View.AccessibilityTraversalAfter.get' is only supported on: 'android' 22.0 and later.

This is an example of calling an API from 22, with a
$(SupportedPlatformOSVersion) for 21.

If <uses-sdk/> is still used in AndroidManifest.xml in a
project, it will be preferred as the final source of truth.

One other improvement is that $(SupportedPlatformOSVersion) is
expected to include .0, or you get the warning:

UnnamedProject.AssemblyInfo.cs(21,12): warning CA1418: Version '21' is not valid for platform 'Android'. Use a version with 2-4 parts for this platform.

Android unique in that this value is always an integer. In order for
users to be able to specify 21, I added a check in
Microsoft.Android.Sdk.DefaultProperties.targets so that we append
.0 when needed.

I added some tests around this scenario.

[dellis1972]

looks ok

[jonathanpeppers]

This broke a lot of tests:

(_CheckForSupportedOSPlatformVersionHigherThanTargetPlatformVersion target) -> 
/Users/runner/Library/Android/dotnet/sdk/6.0.100-rc.1.21372.10/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(185,18):
error MSB4184: The expression "[MSBuild]::VersionGreaterThan(.0, 30.0)" cannot be evaluated. Version string was not in a correct format. 

I'm a bit confused on why I didn't see it locally, looking into it...

[jonathanpeppers]

The problem before was I needed to check if $(SupportedOSPlatformVersion)was blank:

<!-- $(SupportedOSPlatformVersion) must be '21.0', but we should support integer values like '21' -->
<SupportedOSPlatformVersion Condition=" '$(SupportedOSPlatformVersion)' != '' and !$(SupportedOSPlatformVersion.Contains('.')) ">$(SupportedOSPlatformVersion).0</SupportedOSPlatformVersion>

[jaldinger]

I understand this "new" way of specifying the minSdkVersion and targetSdkVersion, however, where can I now specify the targetFrameworkVersion that is used for compiling? The UI in VS2022 (17.3.0 preview 1) does not expose that in a .NET MAUI project, and all of the properties I commented have failed to fix the following warning, also hiding the API methods I require from previous versions:

[jonathanpeppers]

TargetFrameworkVersion should always be 6 for .NET 6. You can see the design about this here:

https://github.com/dotnet/designs/blob/main/accepted/2021/net6.0-tfms/net6.0-tfms.md

For Android class libraries, you can put net6.0-android31 or net6.0-android32 if you really need this. If it is an Android application, I'd recommend putting net6.0-android and let the Android workload choose the default for you.

In the future, you might consider opening an issue instead of commenting on an old PR or comment in the #android channel on Discord. You might get a quicker response that way, thanks!