Fix/suppress bionic google-explicit-constructor warnings

* Add explicit to conversion constructors/operators

Bug: 28341362
Test: make with WITH_TIDY=1 DEFAULT_GLOBAL_TIDY_CHECKS=-*,google-explicit-constructor
Change-Id: Id1ad0327c1b8c6f094bcbb3ae599bc1f716b3f2f
diff --git a/tests/grp_pwd_file_test.cpp b/tests/grp_pwd_file_test.cpp
index 2cbad62..66866fb 100644
--- a/tests/grp_pwd_file_test.cpp
+++ b/tests/grp_pwd_file_test.cpp
@@ -26,7 +26,7 @@
 template <typename T>
 class FileUnmapper {
  public:
-  FileUnmapper(T& file) : file_(file) {
+  explicit FileUnmapper(T& file) : file_(file) {
   }
   ~FileUnmapper() {
     file_.Unmap();