Fix clang-tidy warnings in aapt and aapt2.

* Add explicit keyword to conversion constructors.
* Add NOLINT(implicit) comments for implicit conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters.
Bug: 30407689
* Use const reference type to avoid unnecessary copy.
Bug: 30413862
Test: build with WITH_TIDY=1

Change-Id: Id6d21961f313a1ad92b15a37fdaa5be9e8ab48e1
diff --git a/tools/aapt2/StringPool.h b/tools/aapt2/StringPool.h
index 72ae9d1..13545be 100644
--- a/tools/aapt2/StringPool.h
+++ b/tools/aapt2/StringPool.h
@@ -65,7 +65,7 @@
     private:
         friend class StringPool;
 
-        Ref(Entry* entry);
+        explicit Ref(Entry* entry);
 
         Entry* mEntry;
     };
@@ -88,7 +88,7 @@
     private:
         friend class StringPool;
 
-        StyleRef(StyleEntry* entry);
+        explicit StyleRef(StyleEntry* entry);
 
         StyleEntry* mEntry;
     };