[flexiglass] Adds tracing to bouncer view-models

The scene view-model in BouncerSceneContentViewModel activates its child
view-models depending on the current authentication method (PIN,
password, or pattern). This CL adds a traceCoroutine around that.

Bug: 354270224
Test: changed bouncer auth methods back and forth
Flag: com.android.systemui.scene_container
Change-Id: I51f3b43992d39c8a52afd07ee0d31ecdb5a5b6a0