Move keyguard enabled to Component Override

When using RROs, android:enabled="@bool/form_factor_flag" won't work any
more. So we need to use a component override flag per b/135048762.

Test: Flash an adt3 with RRO enabled. see that systemui is coming up.
Bug: 173138807
Change-Id: If9a99f850ebb34755b4146b786c189f9fb590c3d