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

Bug: 28341362
Change-Id: I14344b0cba134570bf87e7d61aa03503735dd794
Test: build with WITH_TIDY=1
diff --git a/TrimTask.h b/TrimTask.h
index 57be802..2ade7b5 100644
--- a/TrimTask.h
+++ b/TrimTask.h
@@ -27,7 +27,7 @@
 
 class TrimTask {
 public:
-    TrimTask(int flags);
+    explicit TrimTask(int flags);
     virtual ~TrimTask();
 
     enum Flags {