Fix-up offset when transplanting transclusion parts

Symptoms:
* When adding a transclusion part that already exists, if the item is
  being moved to the right it will inserted at a position one place too
  far

Diagnoses:
* When an index is provided with an item in a queue given to
  TransclusionModel's process method, the part is automatically removed,
  but the index, if given, is never adjusted accordingly

Prognosis:
* Cautiously optimistic

Treatment:
* Decrement the insertion index, if given and after the existing index

Change-Id: If321df8f63dae07c76663e76e14864e2f4518920
Merge "Fix-up offset when transplanting transclusion parts"