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.cpp b/init/selinux.cpp
index f897170..8a63363 100644
--- a/init/selinux.cpp
+++ b/init/selinux.cpp
@@ -544,7 +544,7 @@
         PLOG(FATAL) << "restorecon failed of /system/bin/init failed";
     }
 
-    setenv("SELINUX_STARTED_AT", std::to_string(start_time.time_since_epoch().count()).c_str(), 1);
+    setenv(kEnvSelinuxStartedAt, std::to_string(start_time.time_since_epoch().count()).c_str(), 1);
 
     const char* path = "/system/bin/init";
     const char* args[] = {path, "second_stage", nullptr};