embedded.mk: Make phony selinux_policy

Moves selinux policy build decisions to system/sepolicy/Android.mk.
This is done because the PRODUCT_FULL_TREBLE variable isn't available
in embedded.mk and TARGET_SANITIZE isn't available to dependencies of
init.

Test: Build/boot Bullhead PRODUCT_FULL_TREBLE=false
Test: Build/boot Marlin PRODUCT_FULL_TREBLE=true
Test: Build Marlin TARGET_SANITIZE=address. Verify asan rules are
      included in policy output.

Bug: 36138508
Change-Id: I792786cd867bae3e1baeb824167a848948a719df
diff --git a/target/product/embedded.mk b/target/product/embedded.mk
index 681b2c4..a9075c9 100644
--- a/target/product/embedded.mk
+++ b/target/product/embedded.mk
@@ -92,7 +92,8 @@
     plat_mac_permissions.xml \
     plat_property_contexts \
     plat_seapp_contexts \
-    plat_service_contexts
+    plat_service_contexts \
+    selinux_policy
 
 # AID Generation for
 # <pwd.h> and <grp.h>