init.rc: launch update_verifier with exec_start

This allows update_verifier to raise it's priority and ioprio.

Bug: 36511808
Bug: 36102163
Test: Boot bullhead
Test: Verify boottime decrease on sailfish
Change-Id: I5710c6a98dc7acee9063d1fa1d1c80668f0f1528
(cherry picked from commit 1e2d8c7f)