[Xamarin.Android.Build.Tasks] set AndroidApiLevel sooner in <ResolveAndroidTooling/>

[jonathanpeppers]

Context: https://build.azdo.io/3906855

Under a dotnet context, the
GetDependencyWhenBuildToolsAreMissingTest test was failing with:

platforms/android-26 should be a dependency.
  at Xamarin.Android.Build.Tests.StringAssertEx.Contains (System.String text, System.Collections.Generic.IEnumerable`1[T] collection, System.String message) [0x00042] in <8269b6f7f46a4de4813f522c4edb1727>:0
  at Xamarin.Android.Build.Tests.BuildTest.GetDependencyWhenBuildToolsAreMissingTest () [0x000f2] in <8269b6f7f46a4de4813f522c4edb1727>:0

In d7227b8, I partially fixed the GetAndroidDependencies MSBuild
target, but there appears to be another issue.

Reviewing the build log, the GetAndroidDependencies target was
returning:

Done executing task "CalculateProjectDependencies". (TaskId:52)
Target output items:
    platforms/android-21

This was due to the $(_AndroidApiLevel) MSBuild property being
blank, when the <CalculateProjectDependencies/> MSBuild task is
called. It used the $(TargetFrameworkVersion) of v5.0 to decide
that API 21 is needed...

The <ResolveAndroidTooling/> MSBuild task was failing (due to
missing build-tools) and not setting $(_AndroidApiLevel) at all.

To solve this issue, I moved things around so AndroidApiLevel is set
first. Validate can remain an empty method so it is overridden by
the legacy <ResolveAndroidTooling/> task.

Then I hit another problem on Windows:

_ValidateAndroidPackageProperties target) ->
    error XA5207: Could not find android.jar for API level 30. This means the Android SDK platform for API level 30 is not installed. Either install it in the Android SDK Manager (Tools > Android > Android SDK Manager...), or change the Xamarin.Android project to target an API version that is installed. (C:\Program Files (x86)\Android\android-sdk\platforms\android-30\android.jar missing.)

We need this change, I ignored the test on Windows for now:

https://github.com/xamarin/android-sdk-installer/pull/450

This also moves any tests related to the GetAndroidDependencies
MSBuild target to AndroidDependenciesTests. We can now run them all
in a dotnet context.

[jonathanpeppers]

The one failed test is odd:

Xamarin.ProjectTools.FailedBuildException : Build failure: Library1.csprojBuild log recorded at /Users/runner/work/1/s/bin/TestRelease/temp/BuildApplicationWithMonoEnvironmentNormal/Library1/build.log

But there are no errors:

Build succeeded.
    0 Warning(s)
    0 Error(s)

I think dotnet build must have returned a non-zero exit code? Not seen that before, but seems unrelated. We can keep an eye out for it.