Don't dismiss keyguard when enabling it

Introduced in ag/2664156, dismissing a keyguard when it's being disabled has
also caused it to be shown whenever it's being enabled (or re-enabled, or
calling DPM.setKeyguardDisabled when keyguard is enabled).

This change should stop keyguard randomly popping up.

Bug: 133730775
Test: manual
Change-Id: I49f28714b2d149f13168e7564947aaf09c593117