Switch NBShortNumberUtil to a singleton pattern.

This mirrors NBPhoneNumberUtil API and prevents unnecessary resource usage due to loading of metadata.

libPhoneNumberShortNumber/NBShortNumberUtil.h

@@ -25,8 +25,10 @@ typedef NS_ENUM(NSUInteger, NBEShortNumberCost) {
 
 @property(nonatomic) NSDictionary<NSNumber *, NSArray<NSString *> *> *countryToRegionCodeMap;
 
-/// Initialize short number util with a default metadata helper.
-- (instancetype)init;
+/// Short number util singleton with a default metadata helper.
++ (NBShortNumberUtil *)sharedInstance;
+
+- (instancetype)init NS_UNAVAILABLE;
 
 /// Initialize short number util with a metadata helper.
 /// @param helper A metadata helper.

libPhoneNumberShortNumber/NBShortNumberUtil.m

@@ -25,6 +25,17 @@ @implementation NBShortNumberUtil {
   NBPhoneNumberUtil *_phoneUtil;
 }
 
++ (NBShortNumberUtil *)sharedInstance {
+  static NBShortNumberUtil *sharedOnceInstance = nil;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    sharedOnceInstance =
+        [[self alloc] initWithMetadataHelper:[[NBShortNumberMetadataHelper alloc] init]
+                             phoneNumberUtil:[NBPhoneNumberUtil sharedInstance]];
+  });
+  return sharedOnceInstance;
+}
+
 - (instancetype)initWithMetadataHelper:(NBShortNumberMetadataHelper *)helper
                        phoneNumberUtil:(NBPhoneNumberUtil *)phoneNumberUtil {
   self = [super init];
@@ -36,11 +47,6 @@ - (instancetype)initWithMetadataHelper:(NBShortNumberMetadataHelper *)helper
   return self;
 }
 
-- (instancetype)init {
-  return [self initWithMetadataHelper:[[NBShortNumberMetadataHelper alloc] init]
-                      phoneNumberUtil:[NBPhoneNumberUtil sharedInstance]];
-}
-
 - (BOOL)isPossibleShortNumber:(NBPhoneNumber *)phoneNumber forRegion:(NSString *)regionDialingFrom {
   if (![self doesPhoneNumber:phoneNumber matchesRegion:regionDialingFrom]) {
     return NO;