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/ClipArea.h b/libs/hwui/ClipArea.h
index 6eb2eef..32ab501 100644
--- a/libs/hwui/ClipArea.h
+++ b/libs/hwui/ClipArea.h
@@ -97,9 +97,9 @@
 };
 
 struct ClipBase {
-    ClipBase(ClipMode mode)
+    explicit ClipBase(ClipMode mode)
             : mode(mode) {}
-    ClipBase(const Rect& rect)
+    explicit ClipBase(const Rect& rect)
             : mode(ClipMode::Rectangle)
             , rect(rect) {}
     const ClipMode mode;
@@ -112,19 +112,19 @@
 };
 
 struct ClipRect : ClipBase {
-    ClipRect(const Rect& rect)
+    explicit ClipRect(const Rect& rect)
             : ClipBase(rect) {}
 };
 
 struct ClipRectList : ClipBase {
-    ClipRectList(const RectangleList& rectList)
+    explicit ClipRectList(const RectangleList& rectList)
             : ClipBase(ClipMode::RectangleList)
             , rectList(rectList) {}
     RectangleList rectList;
 };
 
 struct ClipRegion : ClipBase {
-    ClipRegion(const SkRegion& region)
+    explicit ClipRegion(const SkRegion& region)
             : ClipBase(ClipMode::Region)
             , region(region) {}
     ClipRegion()