[Java.Interop.Tools.Generator] Improve ExtractApiLevel() resiliency

[jonpryor]

Context: f07b538
Context: https://github.com/jpobst/BindingStudio

While using jpobst/BindingStudio to enumify API-36.1, it would crash:

System.NotSupportedException: Could not parse `W` as an ApiLevel.
   at Java.Interop.Tools.Generator.AndroidSdkVersion.Parse(String value) in W:\src\jpobst\BindingStudio\external\Java.Interop\src\Java.Interop.Tools.Generator\Utilities\AndroidSdkVersion.cs:line 124
   at Java.Interop.Tools.Generator.NamingConverter.ParseApiLevel(String value) in W:\src\jpobst\BindingStudio\external\Java.Interop\src\Java.Interop.Tools.Generator\Utilities\NamingConverter.cs:line 21
   at Java.Interop.Tools.Generator.NamingConverter.ParseApiLevel(XElement element) in W:\src\jpobst\BindingStudio\external\Java.Interop\src\Java.Interop.Tools.Generator\Utilities\NamingConverter.cs:line 60
   at Java.Interop.Tools.Generator.Enumification.MethodMapEntry.FromElement(XElement element, String parameterName) in W:\src\jpobst\BindingStudio\external\Java.Interop\src\Java.Interop.Tools.Generator\Enumification\MethodMapEntry.cs:line 41
   at Java.Interop.Tools.Generator.Enumification.MethodMapEntry.FromElement(XElement element, String parameterName) in W:\src\jpobst\BindingStudio\external\Java.Interop\src\Java.Interop.Tools.Generator\Enumification\MethodMapEntry.cs:line 54
   at Java.Interop.Tools.Generator.Enumification.MethodMapEntry.FromXml(XElement element)+MoveNext() in W:\src\jpobst\BindingStudio\external\Java.Interop\src\Java.Interop.Tools.Generator\Enumification\MethodMapEntry.cs:line 31

The cause of the crash is that the api-36.1.xml file it was processing had //@merge.SourceFile attributes with the value:

merge.SourceFile="…Xamarin-Work/src/dotnet/android/build-tools/create-android-api/../../bin/BuildDebug/api/api-36.1.xml.in"

Notice the -Work in the value. This was used by ExtractApiLevel() to try to find numbers after the -, but W was after the -, not numbers.

This didn't previously matter because f07b538 noted:

Note: Changes to NamingConverter.ParseApiLevel() are largely a
nothing-burger because the dotnet/android side usually has metadata:

    <attr
      api-since="36.1"
      path="/api//*[contains(@merge.SourceFile,'api-CANARY.xml.in')]"
      name="api-since">36.1</attr

which explicitly adds/sets //@api-since based on the
//@merge.SourceFile value.

However, when jpobst/BindingStudio is used, src/Mono.Android/metadata is not used, and //@merge.SourceFile is instead used.

Improve ParseApiLevel() to check for an api- prefix before looking for trailing numeric data.

[jonathanpeppers]

/azp run

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