Refer using std::string instead of global 'string' (#2466)

* Refer using string::std instead of global 'string' i.e to increase portablility, readability and also consistent

* Using 'using' to avoid mentioning namespace everytime

cpp/src/phonenumbers/utf/unicodetext.cc

@@ -27,6 +27,7 @@
 namespace i18n {
 namespace phonenumbers {
 
+using std::string;
 using std::stringstream;
 using std::max;
 using std::hex;