init: move "started at" environment var names to headers

Move "FIRST_STAGE_STARTED_AT" to env_first_stage_started_at variable
in first_stage_init.h.  Move "SELINUX_STARTED_AT" to
env_selinux_started_at variable in selinux.h.

Test: build
Bug: 124491153
Bug: 129780532
Change-Id: I512d1835dcee5a3d034ac453b27b0709eed23cff
diff --git a/init/selinux.h b/init/selinux.h
index 3aa9406..c7d6647 100644
--- a/init/selinux.h
+++ b/init/selinux.h
@@ -35,6 +35,8 @@
                                        const std::vector<std::string>& aliases, int type,
                                        std::string* result);
 
+static constexpr char kEnvSelinuxStartedAt[] = "SELINUX_STARTED_AT";
+
 }  // namespace init
 }  // namespace android