Fix proguard keep rule for constructors.

Proguard expects either the full qualified name or <init> for constructors.

Before (invalid keep rule):
-keep class com.google.android.Foo {
    Foo(java.lang.String);
}

After:
-keep class com.google.android.Foo {
    <init>(java.lang.String);
}

Change-Id: I9d7feaafe112fafb1f58ae534ed915e69033a36c
diff --git a/src/com/google/doclava/Stubs.java b/src/com/google/doclava/Stubs.java
index fa82802..65aa8d8 100644
--- a/src/com/google/doclava/Stubs.java
+++ b/src/com/google/doclava/Stubs.java
@@ -1537,9 +1537,7 @@
 
   static void writeConstructorKeepList(PrintStream keepListWriter, MethodInfo mi) {
     keepListWriter.print("    ");
-    String name = mi.name();
-    name = name.replace(".", "$");
-    keepListWriter.print(name);
+    keepListWriter.print("<init>");
 
     writeParametersKeepList(keepListWriter, mi, mi.parameters());
     keepListWriter.print(";\n");