This is a reland of #178817, which was reverted in #179100 due to test
failures.
The original PR introduced `hitTestBehavior` to the semantics framework
but incorrectly applied `opaque` behavior to `ModalRoute`, which blocked
platform views from receiving pointer events.
Instead of making the entire modal opaque, we:
1. Keep `ModalRoute` without explicit `hitTestBehavior` (defaults to
`defer`)
2. Make only the dialog/sheet content opaque (blocks clicks to barrier)
3. Platform views remain clickable because they're outside the opaque
content boundary
Fixes#149001
Original PR: #177570
Revert: #178744
2a950efec4