Load reboot escrow data at PHASE_BOOT_COMPLETED

Users can't be unlocked before PHASE_BOOT_COMPLETED, as is documented
and enforced in UserController.java.  Therefore, fix
LockSettingsService#loadEscrowData() to run at PHASE_BOOT_COMPLETED
instead of at PHASE_ACTIVITY_MANAGER_READY.  This prevents
LockSettingsService from needing to use Gatekeeper earlier than is
normally the case, which made it more fragile to delays in Gatekeeper
coming up.  It might prevent other problems too.

Bug: 283055837
Test: atest ResumeOnRebootHostTest
Change-Id: I6707b6f33b37f940aecbbd9ec82455fa2a205e99