Add option to disable screen security

See WhisperSystems/TextSecure#788
Merge pull request #794 from SamWhited/issue788

Add option to disable screen security
Update behavior for incoming SMS path.

1) On KitKat, unencrypted SMS messages are never stored in
   TextSecure unless it is set as the system-wide default.

2) On KitKat, if TextSecure is set as the system-wide default,
   provide an option to change the default to a different app.

3) Don't store the TextSecure challenge on KitKat+ devices.
Better support for local storage with passphrase disabled.

1) Never get into a state where messages aren't displayed
   unencrypted.

2) Fix bug where locked DB required launching twice.
make bithub price update // FREEBIE

thanks lilia
pending messages more accurately conveyed to user