[flexiglass] Shortens password textfield focus routing to view-model.

As per the linked comment in the attached bug, we don't need to trigger
an unnecessary additional recomposition when the textfield focus
changes.

Fix: 313940629
Test: manually verified that (a) the IME still comes up when entering
the bouncer scene, (b) dismissing the IME still exits the bouncer scene,
and (c) entering the correct password still correctly unlocks the device
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: If0fb3d5dac3041570406baeb6167d84023a2b87a