[Xamarin.Android.Build.Tasks] require JDK 11 for API 31

[jonathanpeppers]

API 31 builds fail when using JDK 1.8 with:

/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets(1728,3):
error JAVAC0000: java.lang.AssertionError: annotationType() : unrecognized Attribute name MODULE (class com.sun.tools.javac.util.SharedNameTable$NameImpl)

To solve this issue, we must require JDK 11 for API 31.

There are two code paths here for "legacy" Xamarin.Android and .NET 6.
Legacy is straightforward, as we already had a pattern for this.

In .NET 6, I checked the $(TargetPlatformVersion) property for now
and require JDK 11 in that case. When API 31 is stable, we can
probably remove this code and just set this value by default:

<MinimumSupportedJavaVersion Condition=" '$(MinimumSupportedJavaVersion)' == '' ">11.0</MinimumSupportedJavaVersion>

I updated some tests in this area as well.

[jonpryor]

We're starting to wonder if we properly understand what's going on here. The current PR build, which requires JDK 11 for API-31+, has lots of unit test failures, most of which are because of the new check, e.g. LatestTFV_OldTargetSdkVersion:

/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Legacy.targets(239,5):
error XA0031: Java SDK 11.0 or above is required when using $(TargetFrameworkVersion) v11.0.99. 

These tests are currently building now, meaning most of our tests are currently able to target v11.0.99 ("API-S") and build. They're building now, without incurring the JAVAC0000 error about unrecognized Attribute name MODULE.

There's something important that we're not understanding here. :-(

More investigation is required.

[jonathanpeppers]

The tests that fail in #6094 are Xamarin.Forms 4.0 apps that use Android Support libraries.

So I tried doing a Blank Android App in VS 2022, then changed TargetFrameworkVersion=v11.0.99. It also failed with the error.

The problem went away when I removed all AndroidX packages. So perhaps, the presence of Android Support or AndroidX causes the issue? Many of our tests just use plain Android.

[jonathanpeppers]

Ugh, need to fix the conflict

[jonpryor]

API-31 builds fail when using JDK 1.8 and using AndroidX with:

	/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets(1728,3):
	error JAVAC0000: java.lang.AssertionError: annotationType() : unrecognized Attribute name MODULE (class com.sun.tools.javac.util.SharedNameTable$NameImpl)

There are two plausible solutions to better handle this:

 1. Only require JDK 11 when API-31 and AndroidX are being used, or
 2. Always required JDK 11 for API-31+

As we believe *most* apps will be using AndroidX, there is little
point to attempting so support (1); (2) it is!

We must require JDK 11 for API-31+.

There are two code paths here: one for "legacy" Xamarin.Android, and
one for .NET 6.

Legacy is straightforward, as we already had a pattern for this, we
just update the `<ValidateJavaVersion/>` task.

For .NET 6, I checked the `$(TargetPlatformVersion)` property for now
and require JDK 11 in that case.  When API-31 is stable, we can
probably remove this code and just set this value by default:

	<MinimumSupportedJavaVersion Condition=" '$(MinimumSupportedJavaVersion)' == '' ">11.0</MinimumSupportedJavaVersion>

For our unit tests -- most of which would be solution (1) (API-31
and *no* AndroidX use), but many of which started to fail because we
went with solution (2) -- we did one of two things:

 1. Use a different `$(TargetFrameworkVersion)` and/or remove
    `$(TargetFrameworkVersion)` entirely, or

 2. Use a new `BuildTest.AssertTargetFrameworkVersionSupported()`
    method which `Assert.Ignore()`s the test if it would not
    pass otherwise.