Add change id for opting in to R selinux changes

Prepare for allowing apps to opt-in to R changes only.

Bug: 168782947
Test: atest com.android.server.pm.SELinuxMMACTest
Test: atest CompatChangesSelinuxTest
Change-Id: I4b6a1b447917ede00e40dc4162bc01cf4b7f45f2