Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: Ibdd6a210bb7ff228e3624cc319169f77aca3b51e
diff --git a/libs/androidfw/ZipUtils.cpp b/libs/androidfw/ZipUtils.cpp
index 6fa0f14..5abfc8e 100644
--- a/libs/androidfw/ZipUtils.cpp
+++ b/libs/androidfw/ZipUtils.cpp
@@ -150,7 +150,7 @@
 
 class FileReader {
 public:
-   FileReader(FILE* fp) :
+   explicit FileReader(FILE* fp) :
        mFp(fp), mReadBuf(new unsigned char[kReadBufSize])
    {
    }
@@ -170,7 +170,7 @@
 
 class FdReader {
 public:
-   FdReader(int fd) :
+   explicit FdReader(int fd) :
        mFd(fd), mReadBuf(new unsigned char[kReadBufSize])
    {
    }