Allow fixUpInsertion to move offsets when inserting at start/end

fixUpInsertion now returns an object with both data and offset
which allows offset changes.

Within fixUpInsertion we lazy-generate first/lastChildStack which
is a list of parent nodes for which the current node is the first/last
child. Whenever we try to close off a node we check these stacks and
if they are populated we instead use a recursive call to start
fixUpInsertion again but with the offset shifted by 1.

Bug: 46799

Change-Id: Ic51dd03725c11f1f7e279929534ee3afea14d662
Merge "Allow fixUpInsertion to move offsets when inserting at start/end"