SettingsManager: use Flows instead of LiveData

[rfc2822]

Uses Flow instead of LiveData to provide observable settings.

• callbackFlow is used with the SettingsManager.addOnChangeListener.
• The flow always emits the current value (even if it's null; so something like Sync state indicator bar not shown in CALDAV tab #681 won't happen) and then updates
• In the Compose UI, collectAsStateWithLifecycle is used to collect the flow lifecycle-aware: if the Activity is paused, the flow will be stopped; if it's resumed, it will be started again.
• At some points, .asLiveData() is used so that existing code that uses settings LiveData doesn't have to be rewritten in this PR.

In the AccountDetailsPage, there was a problem with the initial value of groupMethod: forcedGroupMethod may be null at the first composition and then be updated. Now updated values of forcedGroupMethod are forced into the groupMethod. In this context, we're also using Flow.map.

In TaskUtils.currentProviderFlow, I have used stateIn to make the SELECTED_TASK_PROVIDER setting flow hot and stateful: the initial value and then incoming updates will be collected to a current state (StateFlow) so that the result always has a correct value.

@ArnyminerZ @sunkup I think this is how we could use Flows in the future (as discussed yesterday). Please test / have a look and tell me your thoughts.

[sunkup]

Comment:
Aha, really nice to have some flow scenarios together in a smaller PR. I see myself coming back taking a look at this in the future whenever I struggle ~

Especially the callback wrapping of our SettingsManager. I wonder whether we could later have SettingsProviders expose their settings as flows as well? I guess it comes down to SharedPreferences exposing settings as flows which will likely never happen, now that DataStore is the recommended approach.

I think the more verbose nature of Flows, in comparison to LiveData, will actually make me become more aware of the android lifecycle mechanisms.

It annoys me a tad, that we need collectAsStateWithLifecycle() and it's so long, but I will get used to it. Other than that I gather flows are much more elegant. Now LiveData feels clumsy in comparison ^^

Review:
It works and looks okay to me. I can't really see any issues with it as of right now.