Knock 0.5s off the Calculator startup time.

Down to 2.5s on mysid. We now only mess around with std::strings when we need
to synthesize a descriptor. If we can just hand out a const char* straight from
the dex file -- which most of the time we can -- we now do.

Change-Id: Iddec7062d8bd578bd25f671eb4d597e9ed064d65
diff --git a/src/utils.cc b/src/utils.cc
index 24e0429..0d6aff9 100644
--- a/src/utils.cc
+++ b/src/utils.cc
@@ -126,8 +126,7 @@
 }
 
 std::string PrettyDescriptor(Primitive::Type type) {
-  char descriptor_char = Primitive::DescriptorChar(type);
-  std::string descriptor_string(1, descriptor_char);
+  std::string descriptor_string(Primitive::Descriptor(type));
   return PrettyDescriptor(descriptor_string);
 }