Prevent content selections ending up inside handlesOwnChildren nodes

The delete logic sees the caption inside the image as a valid place to
put a cursor and therefore a valid range to delete up to. This means
paragraphs can be merged into image captions by backspacing them in.

Add in a flag that keeps track of whether we are in a handlesOwnChildren
node so we can skip over them.

Bug: 57927
Change-Id: I19e91d2d3a871806d207cbd419f4a45e41d69dc0
Merge "Prevent content selections ending up inside handlesOwnChildren nodes"