init: Refactor selinux.h/cpp

This change factors out functions that handle selabels from
selinux.h/cpp into selabel.h/cpp.  This allows util.cpp to be used by
the upcoming native zygote without a bunch of define flags that are
required for selinux.cpp.

Bug: 133443795
Test: Build and boot cuttlefish.
Change-Id: Ie238a96c6407c6698a605dd8803c1727abfaae7b
diff --git a/init/devices.cpp b/init/devices.cpp
index 159c75e..5e760d0 100644
--- a/init/devices.cpp
+++ b/init/devices.cpp
@@ -36,7 +36,7 @@
 #include <selinux/android.h>
 #include <selinux/selinux.h>
 
-#include "selinux.h"
+#include "selabel.h"
 #include "util.h"
 
 #ifdef _INIT_INIT_H