[param-name-importer] Fix crash trying to modify a Dictionary.Values collection directly

[jpobst]

In #756, we switched from using List<JavaPackage> to Dictionary<string, JavaPackage> for better performance. To minimize the diffs needed with existing code, we created an accessor for the Dictionary.Values:

public IDictionary<string, JavaPackage>Packages { get; }
public ICollection<JavaPackage> AllPackages => Packages.Values;

However if you attempt to modify this collection directly, you get an exception.

System.NotSupportedException: Mutating a value collection derived from a dictionary is not allowed.
   at System.ThrowHelper.ThrowNotSupportedException(ExceptionResource resource)
   at Xamarin.Android.ApiTools.JavaStubImporter.JavaStubSourceImporter.ParseJava(String javaSourceText) in C:\code\xamarin-android-backport\external\Java.Interop\tools\param-name-importer\JavaStubSourceImporter.cs:line 81
   at Xamarin.Android.ApiTools.JavaStubImporter.JavaStubSourceImporter.Import(ImporterOptions options) in C:\code\xamarin-android-backport\external\Java.Interop\tools\param-name-importer\JavaStubSourceImporter.cs:line 27
   at Xamarin.Android.ApiTools.Driver.Main(String[] args) in C:\code\xamarin-android-backport\external\Java.Interop\tools\param-name-importer\Program.cs:line 22

Thus we need to update this code that adds to the collection to use the dictionary properly.

Also verified that this is the only code that attempts to write to AllPackages.