Bug/onboarding flag issue

[panasetskaya]

Fix for the bug where onboarding showed on every cold start regardless of the disable_onboarding F flag.

Seems like it was an existing bug that never surfaced since we don't have any other flag that is needed only once on start and is never re-fetched again.

The cause description from Claude:

Root cause was our own bootstrap list. DefaultUnleash.start(bootstrap = …) writes the seeded                                                                      
 toggles into the cache like any other state; readyOnFeaturesReceived() flips ready on the first                                                                   
 non-empty cache update, so bootstrap alone made isReady() true instantly; and                                                                                     
 initializeLocalBackup() only loads the on-disk backup while !ready, so seeding bootstrap also                                                                     
 meant the backup could never load. Net effect: awaitReady() returned immediately with only the                                                                    
 three bootstrapped toggles in cache, and every other flag read false until the first poll landed                                                                  
 ~1s later. OnboardingGate is the app's only one-shot (.first()) flag read that runs at startup,                                                                   
 so it was the only place the defect was visible.                                                                                                                  

So the fix mainly removed bootstrap from isReady(), and did adjustments: added 3 existing defaults for 3 flags to neverFetchedDefaults map consulted only while !client.isReady(), added valueOf(feature), switched featureUpdatedFlow to UnleashStateListener.onStateChanged().

[StylianosGakis]

I think I am mostly concerned with this, if it's equivalent and fits all of our needs here.

I think we can make this a bit more complete with this change

Suggested change

	val listener = object : UnleashStateListener {
	override fun onStateChanged() {
	trySend(Unit)
	}
	}
	val listener = object : UnleashStateListener, UnleashReadyListener {
	override fun onStateChanged() { trySend(Unit) }
	override fun onReady() { trySend(Unit) }
	}