minSdkVersion 21 cannot be smaller than version 23 declared in library [com.firebaseui:firebase-ui-storage:9.0.0]

[thatfiredev]

We bumped the minSdk version of all libraries to 23, but only auth requires minSdk 23.
database, firestore and storage only require minSdk 21.

We should decouple the minSdk in the library and bring database, firestore and storage back to minSdk 21.