Revert "Temporarily revert the SELinux policy for persist.netd.stable_secret."

This re-adds netd_stable_secret_prop to core sepolicy. It was
temporarily reverted so it could be added to device-specific
policy in oc-dr1-dev.

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

This reverts commit 9fa11b77.

Bug: 17613910
Test: make -j64 bootimage
Change-Id: I356c39a5dc955b3d7c28d8c7baf2887a17beb272