24640 Enables semantics when voice control is turned on (cla: yes, platform-ios, waiting for tree to go green)

24644 Fixed problem where AndroidShellHolder was trashing the ThreadHost in its deconstructor (cla: yes, cp: 1.26, platform-android, platform-ios)

24645 fuchsia: Correct PlatformView scale/transform (cla: yes, platform-fuchsia)

24646 Roll Skia from 9fe866174147 to a42c6192ff67 (12 revisions) (cla: yes, waiting for tree to go green)

24647 Roll Dart SDK from a6fb0acd71ea to aa0fea71185b (2 revisions) (cla: yes, waiting for tree to go green)