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
Merged-In: Iab3e6636f60a70cb124f29dc19f20f842fa8dfda
diff --git a/libs/hwui/AssetAtlas.cpp b/libs/hwui/AssetAtlas.cpp
index 2889d2f..037bb0c 100644
--- a/libs/hwui/AssetAtlas.cpp
+++ b/libs/hwui/AssetAtlas.cpp
@@ -29,7 +29,7 @@
 // Lifecycle
 ///////////////////////////////////////////////////////////////////////////////
 
-void AssetAtlas::init(sp<GraphicBuffer> buffer, int64_t* map, int count) {
+void AssetAtlas::init(const sp<GraphicBuffer>& buffer, int64_t* map, int count) {
     if (mImage) {
         return;
     }