android: downgrade targetSdk to 28 (#6961)
android: downgrade targetSdk to 28 (#6961)

Fixes #6953
koreader/koreader,poire-z/koreader,Hzj-jie/koreader,NiLuJe/koreader,mwoz123/koreader,Frenzie/koreader,Markismus/koreader,NiLuJe/koreader,pazos/koreader,Frenzie/koreader,koreader/koreader,poire-z/koreader