Remove the use of Dispachers.Default in AppLockViewModel

[TimoPtr]

Summary

We were using the default dispatcher in the AppLockViewModel but it is not needed since the method called are Main safe already.
Because of the usage of the default dispatcher the unit test of setAppActive is flaky https://github.com/home-assistant/android/actions/runs/24982874143/job/73151266819?pr=6776

[9](https://github.com/home-assistant/android/actions/runs/24982817100/job/73149459136?pr=6777#step:4:590)
> Task :app:testFullDebugUnitTest

AppLockViewModelTest > SetAppActive > Given a null server ID when setting app active then uses SERVER_ID_ACTIVE() FAILED
    java.lang.AssertionError at AppLockViewModelTest.kt:58

858 tests completed, 1 failed

[Copilot]

Pull request overview

This PR removes the explicit use of Dispatchers.Default from AppLockViewModel.setAppActive, allowing the coroutine to use the viewModelScope context (Main dispatcher in production, test dispatcher in unit tests). This aligns the implementation with the fact that the invoked suspend work is already main-safe and eliminates flakiness caused by running on an uncontrolled dispatcher in tests.

Changes:

• Remove the kotlinx.coroutines.Dispatchers import from AppLockViewModel
• Launch setAppActive work in viewModelScope without forcing Dispatchers.Default