Revert "Temporarily remove netd_stable_secret_prop from compat infra."

This reinstates the exception for netd_stable_secret_prop, which
was added after O sepolicy freeze. This exception, along with the
corresponding core sepolicy change, was reverted in order to
allow these policies to be added to per-device sepolicy.

DO NOT SUBMIT until http://ag/2528214 has automerged to master.

This reverts commit 777c8ee0.

Bug: 17613910
Bug: 62573845
Test: make -j64 bootimage
Change-Id: I20b52f1d8e1c0cbb18a339bf45586dacbc7405ad

prebuilts/api/26.0/26.0.ignore.cil

@@ -10,6 +10,7 @@
     hal_tetheroffload_hwservice
     kmsg_debug_device
     memcg_device
+    netd_stable_secret_prop
     sysfs_fs_ext4_features
     timezone_service
     tombstoned_java_trace_socket))