Fix clang-tidy warnings in drm.

* Add explicit keyword to 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: Idf1c48d7bfe3f0da20401cb3ff1983ec00be170d
diff --git a/include/drm/DrmConstraints.h b/include/drm/DrmConstraints.h
index a9ec942..b8bb24f 100644
--- a/include/drm/DrmConstraints.h
+++ b/include/drm/DrmConstraints.h
@@ -70,7 +70,7 @@
     class KeyIterator {
         friend class DrmConstraints;
     private:
-        KeyIterator(DrmConstraints* drmConstraints)
+        explicit KeyIterator(DrmConstraints* drmConstraints)
             : mDrmConstraints(drmConstraints), mIndex(0) {}
 
     public:
@@ -93,7 +93,7 @@
     class Iterator {
         friend class DrmConstraints;
     private:
-        Iterator(DrmConstraints* drmConstraints)
+        explicit Iterator(DrmConstraints* drmConstraints)
             : mDrmConstraints(drmConstraints), mIndex(0) {}
 
     public: