Back left the Manage Libraries sheet open on Android with no way to
dismiss it. The host answered the platform pop with
`BackKeyCoordinator.consumeIfHandled()`, which dedups the focused key
path against the platform pop. Only TV routes one Back through both;
touch platforms never deliver Back to the sheet's key handler, verified
on device — a physical Back produced only `popRoute` and no key event.
So there was nothing to dedup against, and the global one-shot marker,
once set by any other handler, silently swallowed the only signal that
closes the sheet.
Scopes the dedup to TV. The TV regression that guarded this never set
the TV override, so it asserted the swallow on every platform and hid
the defect; it now enables the override and a touch counterpart pins the
dismissal.
Also blocks semantics behind the barrier. The barrier takes every
pointer event but left the screen underneath in the semantics tree, so
assistive tech and UI automation still saw rows that could not be
activated — Maestro read an occluded settings row as visible and tapped
its stale coordinates into the sheet. Flutter's own ModalBarrier blocks
semantics for the same reason.
Verified by replaying the failing Maestro sequence
(.maestro/subflows/settings_deep_checks.yaml lines 42-60) on a device:
back dismisses the sheet, Services opens, and back returns to settings.
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