Delay instantiation of PacProcessor until unlocked

Webkit PAC processor cannot be created before the user is unlocked,
this is currently causing crashes during boot.
With this CL PacProcessor instantiation is delayed and will happen
only when the user is unlocked either when handling a request or
when PAC script is set.

Instantiation of PacProcessor can take substantial time, on order
of a second.

Bug: 200241231
Test: manual, set PAC proxy and reboot
Change-Id: Id42d857289a0b0f7df5b1c48f94d1b363d94235e