Change all exposed MutableLiveData to LiveData

1. Find any public MutableLiveData
2. Rename its variable to prefix `mutable`
3. Make it private
4. Add a public LiveData field to back it

```kotlin
val error = MutableLiveData<String>()
```

to

```kotlin
private val mutableError = MutableLiveData<String>()
val error: LiveData<String> = mutableError
```