adb: shell: add -n flag to not read from stdin.

Shell scripts of the following form do not work properly with adb:
    echo "foo\nbar\nbaz" | {
        read FOO
        while [ "$FOO" != "" ]; do
            adb shell echo $FOO
            read FOO
        done
    }
The first run of adb shell will consume all of the contents of stdin,
causing the loop to immediately end. ssh solves this by providing a -n
flag that causes it to not read from stdin. This commit adds the same.

Bug: http://b/25817224
Change-Id: Id74ca62ef520bcf03678b50f4bf203916fd81038
diff --git a/adb_utils.h b/adb_utils.h
index 537d0e4..388d7dd 100644
--- a/adb_utils.h
+++ b/adb_utils.h
@@ -19,6 +19,8 @@
 
 #include <string>
 
+void close_stdin();
+
 bool getcwd(std::string* cwd);
 bool directory_exists(const std::string& path);