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.

Change-Id: I93214f6ffb8662c238b3351e52bf2bdf23e46a9c
diff --git a/client_library/client_binder.h b/client_library/client_binder.h
index cd857e0..b1b34da 100644
--- a/client_library/client_binder.h
+++ b/client_library/client_binder.h
@@ -53,6 +53,9 @@
                  std::string* out_new_version,
                  int64_t* out_new_size) const override;
 
+  bool SetCohortHint(const std::string& in_cohort_hint) override;
+  bool GetCohortHint(std::string* out_cohort_hint) const override;
+
   bool SetUpdateOverCellularPermission(bool allowed) override;
   bool GetUpdateOverCellularPermission(bool* allowed) const override;