Handle undo in TextView fields with no cursor

This fixes a rare crash in the undo system. In particular, if the
TextView did not have a cursor and the first operation was a
programmatic insert/append the "old cursor position" would be -1.
Attempting to undo would try to restore the cursor to -1 and crash.

Test will land separately in CTS.

Bug: 19332904
Change-Id: I9aa18c1e3621b99d13ac707e483154382effb81c