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.mk b/init/Android.mk
index c4f7d34..0a3e8c7 100644
--- a/init/Android.mk
+++ b/init/Android.mk
@@ -52,6 +52,7 @@
     first_stage_mount.cpp \
     mount_namespace.cpp \
     reboot_utils.cpp \
+    selabel.cpp \
     selinux.cpp \
     switch_root.cpp \
     uevent_listener.cpp \