* make time picker accessible

* use new CustomPaint a11y API

* flutter_localizations tests; use bigger distance delta

* fix am/pm control; selected values

* fix translations; remove @mustCallSuper in describeSemanticsConfiguration

* exclude AM/PM announcement from iOS as on iOS the label is read back automatically

This refactoring allows us to have SemanticsEvent object for events that are not
associated with an accessibility node id.
And allow https://github.com/flutter/flutter/pull/12594 to be a bit
cleaner with a single place for accessibility channel documentation (the
SemanticsEvent classes documentation).

* Send scroll progress with ScrollCompletedSemanticsEvent

This requires engine change https://github.com/flutter/engine/pull/4144

* fix analyze warning

* review comment

* Roll engine to 45b11f742d38ebf564a5a832b1af00661d1a31fa

* fix test

* ++

* ++

* ++

* ++

* ++

* dart docs

* test fix

* undo unintended change

* fix test

* fix test

* review feedback