[Xamarin.Android.Tools.Bytecode] Detect api.xml format doc files (#188)

Originally when `JavaDocletType._ApiXml` was added, I forgot to put
some logic in the method which determines what kind of doclet type to
use from the file path itself, which this addresses.

src/Xamarin.Android.Tools.Bytecode/ClassPath.cs

@@ -240,6 +240,16 @@ JavaDocletType GetDocletType (string path)
 				kind = JavaDocletType.Java7;
 			else if (rawHTML.Contains ("Generated by javadoc (1.8"))
 				kind = JavaDocletType.Java8;
+
+			// Check to see if it's an api.xml formatted doc
+			string rawXML = null;
+			using (var reader = File.OpenText (path)) {
+				int len = reader.ReadBlock (buf, 0, buf.Length);
+				rawXML = new string (buf, 0, len);
+			}
+			if (rawXML.Contains ("<api>") && rawXML.Contains ("<package"))
+				kind = JavaDocletType._ApiXml;
+
 			return kind;
 		}