Update compileSdkVersion 27 to 28

[rabadam]

flutter_appavailability/android/build.gradle:line 25
compileSdkVersion 27

Would it be ok to update to 28?

All my Flutter plugins have compileSdkVersion 28 in their respective build.gradle files.
When generating my Android release build it fails, but succeeds when I manually edit line 25 above to use 28.