pw_string: Rename va_list Format to FormatVaList

On Windows, sometimes the va_list overload would be used when formatting
with a single argument. To avoid potential conflicts like these, the
va_list versions of string::Format and StringBuilder::Format are renamed
to FormatVaList.

Change-Id: Id29fae21b13ed420b13841a479e3748a1d3b91ce
diff --git a/pw_string/format_test.cc b/pw_string/format_test.cc
index 6344ad5..1c44c4e 100644
--- a/pw_string/format_test.cc
+++ b/pw_string/format_test.cc
@@ -76,7 +76,7 @@
 
 TEST(Format, ArgumentLargerThanBuffer_ReturnsResourceExhausted) {
   char buffer[5];
-  auto result = Format(buffer, "%s%c", "2big", '!');
+  auto result = Format(buffer, "%s", "2big!");
 
   EXPECT_EQ(Status::RESOURCE_EXHAUSTED, result.status());
   EXPECT_EQ(4u, result.size());
@@ -87,7 +87,7 @@
   va_list args;
   va_start(args, fmt);
 
-  StatusWithSize result = Format(buffer, fmt, args);
+  StatusWithSize result = FormatVaList(buffer, fmt, args);
 
   va_end(args);
   return result;