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/commandline.cpp b/adb/commandline.cpp
index c69fe19..20d40fd 100644
--- a/adb/commandline.cpp
+++ b/adb/commandline.cpp
@@ -51,6 +51,7 @@
 #include "adb_auth.h"
 #include "adb_client.h"
 #include "adb_io.h"
+#include "adb_unique_fd.h"
 #include "adb_utils.h"
 #include "bugreport.h"
 #include "commandline.h"