Review & Subscribe sheet closes and reopens before the first subscription payment #1467

Describe the bug

Accepting a subscription with an on-chain first payment closes the Review & Subscribe sheet, shows Home for a moment, then opens the Send sheet. Since #1422 the Send sheet shows the same review layout while it pays, so the handoff reads as a flicker of one screen instead of a change between two screens.

Reported by @piotr-iohk in the device QA of #1422, with a recording: #1422 (comment)

Cause

AppViewModel.showSheet clears the current sheet and waits SCREEN_TRANSITION_DELAY before showing the next one. acceptSubscriptionAndStartPayment goes from Sheet.Subscription to Sheet.Send through it.

An in-place replacement for this exact handoff existed and was removed on purpose in 0d9dc5d ("fix: sequence subscription payment sheet", #1186). AppViewModelSendFlowTest asserts the close in initial subscription send dismisses review before presentation. The reason for that change is not recorded in the commit or the PR threads, so #1422 leaves the sequencing as it is.

Expected

The review layout stays on screen from the swipe until Subscribed. SheetHost is a single host that switches content on currentSheet, so a direct swap is possible, provided whatever 0d9dc5d fixed does not come back.

Platform

Android. On iOS the subscription sheet embeds the send flow, so the sheet does not close.

cc @ben-kaufman for the reason behind 0d9dc5d.