[android] stop using Build.VERSION.SdkInt

[jonathanpeppers]

Description of Change

We have a beautiful .NET 6 API we can use instead, that doesn't call
into Java (no JNI!):

https://docs.microsoft.com/dotnet/api/system.operatingsystem.isandroidversionatleast

Usage of Build.VERSION.SdkInt came from both Xamarin.Forms and
Xamarin.Essentials. We were caching the result of the value, but still
doing that work twice.

We can also use the pattern:

OperatingSystem.IsAndroidVersionAtLeast((int)BuildVersionCodes.JellyBean)

BuildVersionCodes is a regular C# enum, and so the C# compiler will
strip this information away leaving just an integer in the final IL.

I used either an integer or the enum, depending on what the existing
code did.

Additionally, I went through any existing code checking old API levels and removed it.
The minimum API level for .NET 6 (dotnet/runtime & xamarin-android) is 21. So any
code looking for API levels 21 or lower, could just be removed.

I also removed code I found such as:

public static int? TargetSdkVersion(this Context self)
{
    return (int?)self?.ApplicationInfo?.TargetSdkVersion;
}

This was only used in one place, and was checking API 17, so I just
removed it.

Results

Testing a Release build of Maui.Controls.Sample.SingleProject.csproj
on a Pixel 5 device:

Before:
12-16 14:41:19.289  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s739ms
12-16 14:41:24.468  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s713ms
12-16 14:41:29.867  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s917ms
12-16 14:41:34.905  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s765ms
12-16 14:41:40.132  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s719ms
12-16 14:41:45.431  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s768ms
12-16 14:41:50.608  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s716ms
12-16 14:41:55.878  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s754ms
12-16 14:42:01.157  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s754ms
12-16 14:42:06.315  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s713ms
Average(ms): 1755.8
Std Err(ms): 19.1803139819046
Std Dev(ms): 60.6534784199921

After:
12-16 14:45:35.208  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s737ms
12-16 14:45:40.448  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s743ms
12-16 14:45:45.663  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s715ms
12-16 14:45:50.931  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s741ms
12-16 14:45:56.161  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s712ms
12-16 14:46:01.426  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s768ms
12-16 14:46:06.574  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s699ms
12-16 14:46:11.867  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s779ms
12-16 14:46:16.979  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s694ms
12-16 14:46:22.229  1741  1935 I ActivityTaskManager: Displayed com.microsoft.maui.sample/crc64dac46b470c4e9200.MainActivity: +1s728ms
Average(ms): 1731.6
Std Err(ms): 8.79924239163047
Std Dev(ms): 27.8256476414596

This appears to maybe save ~24ms on startup for this app?

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the WinUI, Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might affect accessibility?

No

[jonathanpeppers]

NativeVersion.IsAtLeast() and Supports() are public, is it useful at all? Should we delete these methods?

[jonathanpeppers]

Discussed on a call, I'll mark these [Obsolete].

[hartez]

The point of these was to have a cross-platform consistent way for us to do version checking and capabilities. That's why this is a partial class - so we can use these in handlers without having to use #ifdefs.

The intent was to wrap the OperatingSystem calls in the NativeVersion class: #461

Since we've shifted all the of the actual code in the handlers to platform-specific extension methods, maybe these aren't that useful? If that's the case, we don't need to obsolete them - we can just delete them. They're new with MAUI, and haven't seen a stable public release yet.

[jonathanpeppers]

The iOS one seems more useful, as it has this string capability overload:

https://github.com/dotnet/maui/blob/main/src/Core/src/Platform/iOS/NativeVersion.cs

This PR is already getting huge -- so I think I'm going to leave these APIs here and just have them call OperatingSystem on Android.