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/String8.h b/include/utils/String8.h
index 7734cfb..1d12994 100644
--- a/include/utils/String8.h
+++ b/include/utils/String8.h
@@ -17,6 +17,8 @@
 #ifndef ANDROID_STRING8_H
 #define ANDROID_STRING8_H
 
+#include <string> // for std::string
+
 #include <utils/Errors.h>
 #include <utils/Unicode.h>
 #include <utils/TypeHelpers.h>
@@ -63,6 +65,7 @@
     static String8              formatV(const char* fmt, va_list args);
 
     inline  const char*         string() const;
+    static inline std::string   std_string(const String8& str);
     inline  size_t              size() const;
     inline  size_t              bytes() const;
     inline  bool                isEmpty() const;
@@ -261,6 +264,11 @@
     return mString;
 }
 
+inline std::string String8::std_string(const String8& str)
+{
+    return std::string(str.string());
+}
+
 inline size_t String8::size() const
 {
     return length();