Refactor view models to use Kotlin Flows.

[marinacoelho]

Thank you so much @vkryachko for these changes! The services and the view models look great! I checked out the branch and tested all the features, and apart from the two comments I added above, everything is working flawlessly.

A couple of things to keep in mind before merging it:

• This code still does not contain remote config and perf mon, could you pull the latest changes and solve the conflicts? I would like to test the feature flag from rc before merging;
• I added four custom traces to the codebase, 3 of them will be affected by your changes: when creating an account on SignUpViewModel and when saving/updating tasks in EditTaskViewModel. Basically, the trace should start when we call the method from the service, and stop once the result arrives. Let me know if you need help fixing this conflict!

[vkryachko]

I think I've addressed everything, ptal.

As fore tracing I ended up creating a trace() function which I find similar to @AddTrace() in terms of ergonomics, i.e. one can do something like

suspend fun foo() = trace("hello") {
  // do suspend stuff
}

fun bar() = trace("hello") {
  // do stuff
}

wdyt?

[thatfiredev]

@vkryachko you just reminded me that I have a feature request sitting for almost 2 years: https://github.com/firebase/firebase-android-sdk/issues/2029

[vkryachko]

@thatfiredev haha, nice! It never occurred to me to have a standalone function until I tried to use Trace.create("").trace{ } in an app and it felt a bit too verbose :)