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