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.