Reduce StringPiece usage a bit.

Various easy cases, in case we want to move towards making the StringPiece
constructors explicit.

Change-Id: I49e5efc5b787f847ab4cb12d66d1d16aed03fa67
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index f8c342e..23c7416 100644
--- a/src/dex2oat.cc
+++ b/src/dex2oat.cc
@@ -126,7 +126,7 @@
     while (image_classes_file->good()) {
       std::string dot;
       std::getline(*image_classes_file.get(), dot);
-      if (StringPiece(dot).starts_with("#") || dot.empty()) {
+      if (StartsWith(dot, "#") || dot.empty()) {
         continue;
       }
       std::string descriptor(DotToDescriptor(dot.c_str()));