Add {Get,Set}CohortHint interface.

Export the "cohort hint" getter and setter in the client interfaces
(D-Bus and Binder). The cohort hint is sent to Omaha on every update
check request but can be ignored and/or reset by Omaha on every
response.

Other minor linter fixes to the affected files.

Bug: 31740109
Test: Build with D-Bus and with Binder.

(cherry picked from commit 5b5fa8b412312a41cfd4d7ab475b54d4f730ed2a)

Change-Id: Ieb40c2d4bbe2ce926d54b6348b22f2840d8327db
Reviewed-on: https://chromium-review.googlesource.com/395206
Commit-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Christopher Book <cbook@chromium.org>
diff --git a/dbus_service.h b/dbus_service.h
index 12f8cf1..0b63492 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -19,6 +19,7 @@
 
 #include <inttypes.h>
 
+#include <memory>
 #include <string>
 
 #include <base/memory/ref_counted.h>
@@ -91,6 +92,12 @@
                   bool in_get_current_channel,
                   std::string* out_channel) override;
 
+  bool SetCohortHint(brillo::ErrorPtr* error,
+                     const std::string& in_cohort_hint) override;
+
+  bool GetCohortHint(brillo::ErrorPtr* error,
+                     std::string* out_cohort_hint) override;
+
   // Enables or disables the sharing and consuming updates over P2P feature
   // according to the |enabled| argument passed.
   bool SetP2PUpdatePermission(brillo::ErrorPtr* error,