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/Android.bp b/init/Android.bp
index f71437f..232d7e3 100644
--- a/init/Android.bp
+++ b/init/Android.bp
@@ -124,6 +124,7 @@
         "reboot.cpp",
         "reboot_utils.cpp",
         "security.cpp",
+        "selabel.cpp",
         "selinux.cpp",
         "service.cpp",
         "sigchld_handler.cpp",