This app was mentioned in 2 comments, with an average of 1.50 upvotes
This one works for me. It's got a couple different timing settings (japanese byoyomi, canadian byoyomi, and sudden death) and nice, big buttons on either side with a pause button in the middle:
https://play.google.com/store/apps/details?id=com.psocha.goclock&hl=en
Go Game Clock for Android seems to work pretty well for me so far. It handles sudden death, standard byo-yomi, and Canadian byo-yomi (I don't even know what that is). It can be a bit clunky to set up, but not that bad.