As the author of a creative work, you may choose a license by which to authorize others to redistribute your work. And you are not limited to choosing just one license.
If you choose to distribute a library under a given license, it would be inappropriate to not follow the terms of that license yourself in that distribution. So, if you created an app that uses your GPL library, it would be inappropriate to use anything other than the GPL for that app itself. Not doing so would essentially violate a demand you are placing on yourself. And it violates the spirit of the GPL that you licensed your library under.
However, if you wanted to release your app under a proprietary or different license, you can also dual-license your GPL library, and use the proprietarily licensed version for your app, and the GPL version for everyone else. Trying to single-license the library per the GPL and use it in your proprietary app is a murky land of unknown law. The complication comes from trying to identify the license of the application that has the bundled library.
Say I will create an android app that uses the aforementioned GPL library. Do I need to publish it under GPL aswell?
So, you don't really have to, but you should, and the consequences if you don't are strange and unclear.