Fix google-explicit-constructor warnings in libs/hwui.

* Add explicit keyword to conversion constructors,
  or add NOLINT for implicit converters.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: Id8ca42433a4fc3652e4cb13787c4cb169f20d9a9
diff --git a/libs/hwui/GpuMemoryTracker.h b/libs/hwui/GpuMemoryTracker.h
index 851aeae..bfb1bf1 100644
--- a/libs/hwui/GpuMemoryTracker.h
+++ b/libs/hwui/GpuMemoryTracker.h
@@ -52,7 +52,7 @@
     static void onFrameCompleted();
 
 protected:
-    GpuMemoryTracker(GpuObjectType type) : mType(type) {
+    explicit GpuMemoryTracker(GpuObjectType type) : mType(type) {
         ASSERT_GPU_THREAD();
         startTrackingObject();
     }