Cleanup - Remove invalid onStart

The animation onStart() is a side-effecting function, returning
Unit. Attempting to return a value here is invalid.

Bug: 322198222
Test: atest com.android.systemui.keyguard
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
STAGING
Flag: ACONFIG com.android.systemui.migrate_clocks_to_blueprint
DEVELOPMENT
Flag: ACONFIG com.android.systemui.keyguard_bottom_area_refactor
STAGING

Change-Id: Ic8ecf64c31918cc41e7533bf5c646adb8f6674fc