Fixed KDF iterations to be honored, and other bugs.

1. PRAGMA kdf_iter wasn't happening at the right point, so we were using
the default 65k iterations. Fixed.
2. Upped iterations to 256k after some testing on a Nexus 5. This might
be ungainly...
3. Removed unnecessary SQLDatabaseHelper usage and simplified
SQLCipherDatabase design.
4. Minor bugfix in Common DisplayException().