Preserve/restore calculator instance state

Bug: 15597963
Bug: 16850193

- Manage tokenizer instance at activity level so it is re-created on
locale changes.
- Preserve normalized form of expression in instance state and
restore/evaluate on activity creation.
- Remove selection hack from CalculatorEditText since setting a selection
makes TextView save instance state and it no longer appears necessary.

Change-Id: Ic15924f4026ec45b4775b36568c3cdc46443cca8