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/util.cpp b/init/util.cpp
index 63d2d44..243e5f0 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -41,7 +41,7 @@
 
 #if defined(__ANDROID__)
 #include "reboot_utils.h"
-#include "selinux.h"
+#include "selabel.h"
 #else
 #include "host_init_stubs.h"
 #endif