libutils: add String8 and String16 std_string utility methods

Add utility methods that convert String16 and String8 into
std::string.

Also, remove a repeated include of <utils/Unicode.h> in
String16.h, since it is not used in that header file,
and is already included in String16.cpp.

BUG: 27200800
Change-Id: I5238aeb70689499763060a99dff9950fbb7adb3e
TEST: libutils builds successfully.
diff --git a/include/utils/String16.h b/include/utils/String16.h
index 50ac6d0..4a5874a 100644
--- a/include/utils/String16.h
+++ b/include/utils/String16.h
@@ -17,8 +17,10 @@
 #ifndef ANDROID_STRING16_H
 #define ANDROID_STRING16_H
 
+#include <string> // for std::string
+
 #include <utils/Errors.h>
-#include <utils/Unicode.h>
+#include <utils/String8.h>
 #include <utils/TypeHelpers.h>
 
 // ---------------------------------------------------------------------------
@@ -65,6 +67,7 @@
 
     inline  const char16_t*     string() const;
 
+    static inline std::string   std_string(const String16& str);
             size_t              size() const;
             void                setTo(const String16& other);
             status_t            setTo(const char16_t* other);
@@ -142,6 +145,11 @@
     return mString;
 }
 
+inline std::string String16::std_string(const String16& str)
+{
+    return std::string(String8(str).string());
+}
+
 inline String16& String16::operator=(const String16& other)
 {
     setTo(other);