The arrow key and delete handling in RenderEditable was using single
index values, which made it possible to move the cursor into the middle
of a surrogate pair (allowing things like adding text at that insertion
point), or to delete half of a surrogate pair. Since unpaired surrogate
pairs aren't valid UTF-16, doing so would cause assertions in the text
field.

This makes the arrow key and delete key handling surrogate-aware
(although not grapheme-cluster-aware, which is a larger fix that is out
of scope here).

Part of #55014

ef05a18c0 Roll src/third_party/dart 87b829bacd36..e6baa97e9adc (5 commits) (flutter/engine#17870) (#55368)

4616931ea Roll src/third_party/skia e9663db508df..a6cd5588d231 (4 commits) (flutter/engine#17868) (#55363)

Prefer using `.of()` (which is statically type-checked) over
`.from()` (which is dynamically type-checked) when possible.

* e83f30c59 Pass amber-files directory to tests. (flutter/engine#17815)

* b1c51cfe7 Extend external view embedder on Android (flutter/engine#17839)

cf78b89c3 Roll src/third_party/skia 24ea293cd25b..efb2133b0dd3 (7 commits) (flutter/engine#17847) (#55314)

* disable hit testing if the CompositedTransformFollower is hidden when there is no leader layer

* update comment