SL composable/viewmodel cleanup

Moves comment to public variable, updates visibility modifiers for scope,
adds test for secure lock device viewmodel, removes applicationscope from
viewmodel, change mutable state flows to snapshot state where possible for
performance

Flag: android.security.secure_lock_device
Bug: 401645997
Test: atest PromptViewModelTest
Test: atest SecureLockDeviceBiometricAuthContentViewModelTest
Test: atest BiometricAuthIconViewModelTest
Change-Id: I2e55a66872d6be797b194853a1d9a160fa75619c