Address code warnings in the android.app package.

- Make fields that are only assigned once private.
- Fix typos.
- Remove unnecessary boxed types.
- Remove unused default value variable assignemnts.
- Merge repetitive exception handling.

Test: m dist -j && acloud create --local-instance --local-image
Change-Id: Idc2aa16798d6cf4fe9df0a13f37e002667a1de41