This app was mentioned in 6 comments, with an average of 1.00 upvote
Decline is a shooter which meets all of your criteria (closed source for now, but is free with minimal ad support). Give it a try and feel free to let me know what you think.
DECLINE
Infinite retro-themed shmup.
Available for Android in the play store (free): https://play.google.com/store/apps/details?id=com.grep.decline
Interested in feedback because I'm not sure what I need to do to make this more appealing.
Cheers.