Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I2cf746fd07addc6e6c22e98e2a5bb8a8ce47c885

logcat/tests/logcat_benchmark.cpp

@@ -49,7 +49,7 @@ TEST(logcat, sorted_order) {
             }
         }
 
-        timestamp(const char *buffer)
+        explicit timestamp(const char *buffer)
         {
             init(buffer);
         }

logd/LogBuffer.cpp

@@ -380,7 +380,7 @@ public:
             tid(tid),
             padding(0) {
     }
-    LogBufferElementKey(uint64_t key):value(key) { }
+    explicit LogBufferElementKey(uint64_t key):value(key) { }
 
     uint64_t getKey() { return value; }
 };