Fix Can't Able to Click Sims

The root cause is SubscriptionManager.OnSubscriptionsChangedListener
.onSubscriptionsChanged() not invoked in some cases.

Even the SubscriptionManager.addOnSubscriptionsChangedListener's doc
says the onSubscriptionsChanged() method will also be invoked once
initially when calling it, there still case that the
onSubscriptionsChanged() method is not invoked initially.
For example, when the onSubscriptionsChanged event never happens before,
on a device never ever has any subscriptions.

Adding a .onStart { emit(Unit) } to fix.

Also make the subscriptionsChangedFlow() a shared flow to mitigate the
extra emit cost.

Bug: 369276595
Flag: EXEMPT bug fix
Test: manual - factory reset & no any sim
Test: atest SubscriptionRepositoryTest
Change-Id: Ic32a5666f14373926b5dfedb5dedadb4369acfc7