Fix google-explicit-constructor warnings in system/update_engine.

Declare explicit conversion constructors.

Bug: 28341362
Change-Id: I92ac0d1faf6996de1b882ff13a2f44a3abbb787f
Test: build with clang-tidy
diff --git a/binder_service_android.h b/binder_service_android.h
index 47e76a7..3fb38bc 100644
--- a/binder_service_android.h
+++ b/binder_service_android.h
@@ -36,7 +36,7 @@
 class BinderUpdateEngineAndroidService : public android::os::BnUpdateEngine,
                                          public ServiceObserverInterface {
  public:
-  BinderUpdateEngineAndroidService(
+  explicit BinderUpdateEngineAndroidService(
       ServiceDelegateAndroidInterface* service_delegate);
   ~BinderUpdateEngineAndroidService() override = default;