Protect against offset=-1 in insertContent()

The annotation code here is much much too simplistic.
We probably want to either use the insertionAnnotations,
or factor out the code that computes the insertionAnnotations
in ve.dm.Surface#setSelection.

Bug: 72558
Change-Id: I6cb35e12e92d8adc06453f87a3596ef8b57ec54f
Merge "Protect against offset=-1 in insertContent()"