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/fake_filesystem.cc b/payload_generator/fake_filesystem.cc
index 494792d..3a6458d 100644
--- a/payload_generator/fake_filesystem.cc
+++ b/payload_generator/fake_filesystem.cc
@@ -50,7 +50,7 @@
   files_.push_back(file);
 }
 
-bool FakeFilesystem::LoadSettings(chromeos::KeyValueStore* store) const {
+bool FakeFilesystem::LoadSettings(brillo::KeyValueStore* store) const {
   if (minor_version_ < 0)
     return false;
   store->SetString("PAYLOAD_MINOR_VERSION", std::to_string(minor_version_));