Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: Ibdd6a210bb7ff228e3624cc319169f77aca3b51e
diff --git a/native/android/asset_manager.cpp b/native/android/asset_manager.cpp
index dee3f8c..0e5e5c6 100644
--- a/native/android/asset_manager.cpp
+++ b/native/android/asset_manager.cpp
@@ -39,7 +39,7 @@
     size_t mCurFileIndex;
     String8 mCachedFileName;
 
-    AAssetDir(AssetDir* dir) : mAssetDir(dir), mCurFileIndex(0) { }
+    explicit AAssetDir(AssetDir* dir) : mAssetDir(dir), mCurFileIndex(0) { }
     ~AAssetDir() { delete mAssetDir; }
 };
 
@@ -48,7 +48,7 @@
 struct AAsset {
     Asset* mAsset;
 
-    AAsset(Asset* asset) : mAsset(asset) { }
+    explicit AAsset(Asset* asset) : mAsset(asset) { }
     ~AAsset() { delete mAsset; }
 };