update_verifier: raise priority and ioprio and start with exec_start

Raise the priority and ioprio of update_verifier and launch with
exec_start.  This saves ~100ms of time before `class_start main` is executed.

Bug: 36511808
Bug: 36102163
Test: Boot bullhead
Test: Verify boottime decrease on sailfish
Change-Id: I944a6c0d4368ead5b99171f49142da2523ed1bdd
(cherry picked from commit 545317f4)