Fix clang-tidy warnings in libs/hwui.

* Add explicit keyword to conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters.
Bug: 30407689
Test: build with WITH_TIDY=1

Change-Id: Iab3e6636f60a70cb124f29dc19f20f842fa8dfda
diff --git a/libs/hwui/utils/Timing.h b/libs/hwui/utils/Timing.h
index dd8847a..4b1fabe 100644
--- a/libs/hwui/utils/Timing.h
+++ b/libs/hwui/utils/Timing.h
@@ -22,7 +22,7 @@
 #define TIME_METHOD() MethodTimer __method_timer(__func__)
 class MethodTimer {
 public:
-    MethodTimer(const char* name)
+    explicit MethodTimer(const char* name)
             : mMethodName(name) {
         gettimeofday(&mStart, nullptr);
     }