Fix google-explicit-constructor warnings in bootable/recovery.

Bug: 28341362
Change-Id: I4c3ab0ad0f62f8f3a6f3b30771010533a2739381
Test: build with clang-tidy
diff --git a/device.h b/device.h
index f74b6b0..e0a3ff7 100644
--- a/device.h
+++ b/device.h
@@ -21,7 +21,7 @@
 
 class Device {
   public:
-    Device(RecoveryUI* ui) : ui_(ui) { }
+    explicit Device(RecoveryUI* ui) : ui_(ui) { }
     virtual ~Device() { }
 
     // Called to obtain the UI object that should be used to display