adb: extract the adb-specific unique_fd out into its own header.

Some POSIX-only files would like to use functions declared in
adb_utils.h without being forced to use the sysdeps stuff for open,
close, etc. Extract the adb-specific unique_fd into its own header so
those files can use adb_utils.h alongside the real unique_fd.

Change-Id: I10344ef26b7e72fce46bf7e3033f89aa54bd6118
Test: mm
diff --git a/adb/adb_utils.cpp b/adb/adb_utils.cpp
index e138a90..db39ef4 100644
--- a/adb/adb_utils.cpp
+++ b/adb/adb_utils.cpp
@@ -17,6 +17,7 @@
 #define TRACE_TAG ADB
 
 #include "adb_utils.h"
+#include "adb_unique_fd.h"
 
 #include <libgen.h>
 #include <stdlib.h>
@@ -309,3 +310,7 @@
     }
     return android_dir;
 }
+
+void AdbCloser::Close(int fd) {
+    adb_close(fd);
+}