This allows plain text pasting into source mode without interfering with the
possible-conversion that should happen when it's pasted into visual mode.
Bug: T190590
Bug: T192320
Depends-On: I47a5bf321fb64d4a631ec6fd728bee269d4cdae0
Change-Id: I71db368c683a6a846569d7627f4cc73e907a61d2