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/ResourceUtils.cpp b/tools/aapt2/ResourceUtils.cpp
index 7dc88ded..773616d 100644
--- a/tools/aapt2/ResourceUtils.cpp
+++ b/tools/aapt2/ResourceUtils.cpp
@@ -545,7 +545,7 @@
 std::unique_ptr<Item> parseItemForAttribute(
         const StringPiece& value,
         uint32_t typeMask,
-        std::function<void(const ResourceName&)> onCreateReference) {
+        const std::function<void(const ResourceName&)>& onCreateReference) {
     std::unique_ptr<BinaryPrimitive> nullOrEmpty = tryParseNullOrEmpty(value);
     if (nullOrEmpty) {
         return std::move(nullOrEmpty);
@@ -604,7 +604,7 @@
  */
 std::unique_ptr<Item> parseItemForAttribute(
         const StringPiece& str, const Attribute* attr,
-        std::function<void(const ResourceName&)> onCreateReference) {
+        const std::function<void(const ResourceName&)>& onCreateReference) {
     const uint32_t typeMask = attr->typeMask;
     std::unique_ptr<Item> value = parseItemForAttribute(str, typeMask, onCreateReference);
     if (value) {