AU: Add support for persistent update engine preferences store.

BUG=5140
TEST=emerge-x86-generic, attached unit tests

Review URL: http://codereview.chromium.org/3014035
diff --git a/SConstruct b/SConstruct
index 216eb1d..cf51f47 100644
--- a/SConstruct
+++ b/SConstruct
@@ -214,6 +214,7 @@
                    omaha_request_params.cc
                    omaha_response_handler_action.cc
                    postinstall_runner_action.cc
+                   prefs.cc
                    set_bootable_flag_action.cc
                    simple_key_value_store.cc
                    split_file_writer.cc
@@ -247,6 +248,7 @@
                             omaha_request_params_unittest.cc
                             omaha_response_handler_action_unittest.cc
                             postinstall_runner_action_unittest.cc
+                            prefs_unittest.cc
                             set_bootable_flag_action_unittest.cc
                             simple_key_value_store_unittest.cc
                             split_file_writer_unittest.cc