Cleaning up some app model code.

- Preventing extra sorting when adding/updating apps
- Preventing extra logic when filtering apps
- Fixing overlapping prediction bar when all predictions are removed
- Fixing crash when retrieving section names for AppInfos whose titles
  have been updated, but updateApps has not yet been called.

Change-Id: I1da468b0fd5c5cc404b6a5e6146a268fefeca267