There are two separate issues here, code security and database security.
For all of your applications you will probably want to use ProGaurd in order to obfuscate your code so that others cannot view its original form. If someone attempts to decompile your APK then the result will be a bunch of gibberish that cannot be understood. ProGuard is relatively simple to setup especially with Android Studio. For Eclipse users look at this question.
Anyone with a rooted Android phone can browse through the file system and copy your database, then open it up and view all of the data. In order to avoid this you will want to encrypt your db. Check out this question for a place to start off with database encryption.