This updates the FlutterSemanticsAction enumerator identifiers for the
'move cursor forward/back one word' actions (added in
flutter/engine#8033) for consistency with the 'move cusor forward/back
on character' identifiers.

ABI compatibility is unaffected, but this with require the following
change in any embedder making use of these fields:

Rename:
    kFlutterSemanticsActionMoveCursorForwardByWordIndex
to:
    kFlutterSemanticsActionMoveCursorForwardByWord

Rename:
    kFlutterSemanticsActionMoveCursorBackwardByWordIndex
to:
    kFlutterSemanticsActionMoveCursorBackwardByWord