I completed half of an android MOOC course and for the most part it was just Java + mobile design patterns.
The android specific stuff (build tools/IDE, configuration, version partitioning, etc) can be learned on the fly I assume.
As someone who has developed an app for Android (going into it knowing Java really well) and now porting it to iOS using swift ( not knowing Swift at all), the language is actually the least of your worries. It's like you say the tooling, the paradigms, the UI and libraries, SQLite etc which are the biggest learning challenges. Also mobile dev is unique in its self (limited hardware & no reliable network conn)
I completed half of an android MOOC course and for the most part it was just Java + mobile design patterns. The android specific stuff (build tools/IDE, configuration, version partitioning, etc) can be learned on the fly I assume.