buffet: Move command state into CommandInstance

The command state is now stored in CommandInstance and not in
DBusCommandProxy. CommandInstance can now notify the proxy of
command state changes via CommandProxyInterface.

Moved command status strings from dbus_constants.h into the
CommandInstance class, as members.

Added a property on DBusCommandProxy to expose the command
parameters to command handlers, so they can get the parameter
values over D-Bus.

BUG=chromium:374864
TEST=FEATURES=test emerge-link buffet

Change-Id: Ief3397ef09644772ffc3b1b01ed972a8b6779df4
Reviewed-on: https://chromium-review.googlesource.com/216296
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/dbus_constants.cc b/buffet/dbus_constants.cc
index 7fc0acf..c3921f9 100644
--- a/buffet/dbus_constants.cc
+++ b/buffet/dbus_constants.cc
@@ -35,15 +35,7 @@
 const char kCommandId[] = "Id";
 const char kCommandStatus[] = "Status";
 const char kCommandProgress[] = "Progress";
-
-const char kCommandStatusQueued[] = "queued";
-const char kCommandStatusInProgress[] = "inProgress";
-const char kCommandStatusPaused[] = "paused";
-const char kCommandStatusError[] = "error";
-const char kCommandStatusDone[] = "done";
-const char kCommandStatusCanceled[] = "canceled";
-const char kCommandStatusAborted[] = "aborted";
-const char kCommandStatusExpired[] = "expired";
+const char kCommandParameters[] = "Parameters";
 
 }  // namespace dbus_constants