update_engine: Rename "chromeos" -> "brillo" in include paths and namespaces

libchromeos is transitioning to libbrillo and chromeos namespaces
and include directory is changing to brillo.

Bug: 24872993
Change-Id: I770659a95be380a50fe3b2ba9f91d65818f40945
diff --git a/payload_generator/ext2_filesystem.h b/payload_generator/ext2_filesystem.h
index afbe524..248e208 100644
--- a/payload_generator/ext2_filesystem.h
+++ b/payload_generator/ext2_filesystem.h
@@ -52,7 +52,7 @@
   //    and bitmap tables.
   bool GetFiles(std::vector<File>* files) const override;
 
-  bool LoadSettings(chromeos::KeyValueStore* store) const override;
+  bool LoadSettings(brillo::KeyValueStore* store) const override;
 
  private:
   Ext2Filesystem() = default;