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/UpdateEngine.conf b/UpdateEngine.conf
index 9238e3a..58cca09 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -38,6 +38,12 @@
            send_member="GetChannel"/>
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
+           send_member="SetCohortHint"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="GetCohortHint"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
            send_member="SetP2PUpdatePermission"/>
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"