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/binder_bindings/android/brillo/IUpdateEngine.aidl b/binder_bindings/android/brillo/IUpdateEngine.aidl
index 6a3295a..b1a1b4f 100644
--- a/binder_bindings/android/brillo/IUpdateEngine.aidl
+++ b/binder_bindings/android/brillo/IUpdateEngine.aidl
@@ -28,6 +28,8 @@
   void RebootIfNeeded();
   void SetChannel(in String target_channel, in boolean powewash);
   String GetChannel(in boolean get_current_channel);
+  void SetCohortHint(in String cohort_hint);
+  String GetCohortHint();
   void SetP2PUpdatePermission(in boolean enabled);
   boolean GetP2PUpdatePermission();
   void SetUpdateOverCellularPermission(in boolean enabled);