We're using the default package manager for Android studio, which is gradle.
Make sure you have internet connection when compiling for the first time
https://docs.gradle.org/current/userguide/dependency_management.html#gsc.tab=0
Upon your first launch on the project, make sure you sync gradle just to make sure packages are installed.
We're using API level 21, or Android v5.0 lollipop due to Android Beam and all the new Bluetooth features added. Plus, the targeted market size is quite large.