Rename big/small stack to undoStack & newTransactions

Becuase big/small stack doesn't really mean anything.
Bonus: rename the transactions property of the stack item from
'stack' to 'transactions'.

Bug: 49754
Change-Id: I361dda49f4c1e58a541df5b9e478cf20957939a1
Merge "Rename big/small stack to undoStack & newTransactions"