Add private constructors to utility classes

See <https://stackoverflow.com/q/1844355>.

Author: yawnoc

app/src/main/java/io/github/yawnoc/utilities/Contexty.java

@@ -16,6 +16,10 @@
 
 public final class Contexty {
 
+  private Contexty() {
+    // Do not instantiate
+  }
+  
   public static String loadPreferenceString(
     final Context context,
     final String preferenceFileName,

app/src/main/java/io/github/yawnoc/utilities/Mappy.java

@@ -12,6 +12,10 @@
 
 public class Mappy {
 
+  private Mappy() {
+    // Do not instantiate
+  }
+  
   public static <V, K> Map<V, K> invertMap(Map<K, V> forwardMap) {
 
     Map<V, K> inverseMap = new HashMap<>();

app/src/main/java/io/github/yawnoc/utilities/Stringy.java

@@ -9,6 +9,10 @@
 
 public class Stringy {
 
+  private Stringy() {
+    // Do not instantiate
+  }
+  
   public static boolean isAscii(final String string) {
     return string.matches("\\p{ASCII}*");
   }

app/src/main/java/io/github/yawnoc/utilities/Valuey.java

@@ -25,6 +25,10 @@
 
 public class Valuey {
 
+  private Valuey() {
+    // Do not instantiate
+  }
+  
   public static int getDimensionOrFraction(
     final TypedArray array,
     final int attributeIndex,