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/DrmInfoRequest.h b/include/drm/DrmInfoRequest.h
index 3e48ecc..e2da4fe 100644
--- a/include/drm/DrmInfoRequest.h
+++ b/include/drm/DrmInfoRequest.h
@@ -69,7 +69,7 @@
         friend class DrmInfoRequest;
 
     private:
-        KeyIterator(const DrmInfoRequest* drmInfoRequest)
+        explicit KeyIterator(const DrmInfoRequest* drmInfoRequest)
             : mDrmInfoRequest(const_cast <DrmInfoRequest*> (drmInfoRequest)), mIndex(0) {}
 
     public:
@@ -93,7 +93,7 @@
         friend class DrmInfoRequest;
 
     private:
-        Iterator(const DrmInfoRequest* drmInfoRequest)
+        explicit Iterator(const DrmInfoRequest* drmInfoRequest)
             : mDrmInfoRequest(const_cast <DrmInfoRequest*> (drmInfoRequest)), mIndex(0) {}
 
     public: