[generator] less aggressive obfuscation marking.

This is a fixup for c51469e, as well as bugfix for:
https://bugzilla.xamarin.com/show_bug.cgi?id=51337

What's happening behind bug #51337 was this:

	BINDINGSGENERATOR:  warning BG8800: Unknown parameter
	type org.osmdroid.ResourceProxy.bitmap in method GetBitmap
	in managed type OsmDroid.DefaultResourceProxyImpl.

This was, because, "bitmap" is all lowercased and was marked as obfuscated
(because, WHY NAME A CLASS ALL IN LOWERCASE!?).

Such classes should not exist, or should be marked as "obfuscated='false'".

However what bug #51337 implies is that people are not going to make
this additional markup for generator improvements.

What this generator change does is then - a hack. A hack to mark "classes
with very short names" as obfuscated, instead of "all lowercase or number".

As commented on c51469e, there isn't good way to check API element
siblings (it can check sibling names every time at all expensive
calculation). So we just count the sibling nodes and if name length is
short enough to fit within the number of classes, we mark as obfucated.

As some post-first-commit thought, the name check is done only for
very short names i.e. only those within 3 letters.

Also, the new proguard in Gradle task seems to obfuscates the names as
"zzXXXXX" . They should be marked as obfuscated too.

Author: atsushieno

tools/generator/GenBaseSupport.cs

@@ -204,7 +204,7 @@ public XmlGenBaseSupport (XmlElement pkg, XmlElement elem)
 				full_name = String.Format ("{0}.{1}{2}", ns, idx > 0 ? StringRocks.TypeToPascalCase (java_name.Substring (0, idx + 1)) : String.Empty, name);
 			}
 
-			obfuscated = IsObfuscatedName (java_name) && elem.XGetAttribute ("obfuscated") != "false";
+			obfuscated = IsObfuscatedName (pkg.ChildNodes.Count, java_name) && elem.XGetAttribute ("obfuscated") != "false";
 		}
 
 		public override bool IsAcw {
@@ -302,12 +302,21 @@ public override bool ValidateNamespace ()
 			return true;
 		}
 
-		bool IsObfuscatedName (String name)
+		bool IsObfuscatedName (int threshold, string name)
 		{
 			if (name.StartsWith ("R.", StringComparison.Ordinal))
 				return false;
 			int idx = name.LastIndexOf ('.');
 			string last = idx < 0 ? name : name.Substring (idx + 1);
+			// probably new proguard within Gradle tasks, used in recent GooglePlayServices in 2016 or later.
+			if (last.StartsWith ("zz", StringComparison.Ordinal))
+				return true;
+			// do not expect any name with more than 3 letters is an 'obfuscated' one.
+			if (last.Length > 3)
+				return false;
+			// Only short ones ('a', 'b', 'c' ... 'aa', 'ab', ... 'zzz') are the targets.
+			if (!(name.Length == 3 && threshold > 26*26 || name.Length == 2 && threshold > 26 || name.Length == 1))
+				return false;
 			if (last.Any (c => (c < 'a' || 'z' < c) && (c < '0' || '9' < c)))
 					return false;
 			return true;