Store UUIDs as 16 bytes of data

This makes them much more efficient to write over shared memory to solve
the boot UUID logging problem when we add them.

Change-Id: Idf361d6b096bfa52cbc98f555c90bf1f6b90d3e0
diff --git a/aos/events/logging/log_namer.cc b/aos/events/logging/log_namer.cc
index 44e8f5a..18138d9 100644
--- a/aos/events/logging/log_namer.cc
+++ b/aos/events/logging/log_namer.cc
@@ -20,11 +20,10 @@
     aos::SizePrefixedFlatbufferDetachedBuffer<LogFileHeader> *header,
     const UUID &uuid, int parts_index) const {
   header->mutable_message()->mutate_parts_index(parts_index);
-  CHECK_EQ(uuid.string_view().size(),
+  CHECK_EQ(UUID::kStringSize,
            header->mutable_message()->mutable_parts_uuid()->size());
-  std::copy(uuid.string_view().begin(), uuid.string_view().end(),
-            reinterpret_cast<char *>(
-                header->mutable_message()->mutable_parts_uuid()->Data()));
+  uuid.CopyTo(reinterpret_cast<char *>(
+      header->mutable_message()->mutable_parts_uuid()->Data()));
 }
 
 void LocalLogNamer::WriteHeader(