Fix google-explicit-constructor warnings in common_time.

* Add explicit keyword to conversion constructors.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: Ieab4756042e77cfadda35e34f78f515058ae0bb3
diff --git a/libs/common_time/common_time_config_service.h b/libs/common_time/common_time_config_service.h
index 89806dd..23abb1a 100644
--- a/libs/common_time/common_time_config_service.h
+++ b/libs/common_time/common_time_config_service.h
@@ -49,7 +49,7 @@
     virtual status_t forceNetworklessMasterMode();
 
   private:
-    CommonTimeConfigService(CommonTimeServer& timeServer)
+    explicit CommonTimeConfigService(CommonTimeServer& timeServer)
         : mTimeServer(timeServer) { }
     CommonTimeServer& mTimeServer;