fix android 28 no toast bug

[xinwii]

update target version to 28

[chankruze]

This is a library for android application.There is no need of this app.
These are handled by the project app gradle itself.

By the way everything seems to be ok.But only updating target android version don't make the app compatible. But in this case it will work.

This gradle change can fix all the failed checks for travis ci (#35 )

[lucazin]

I have compileSDK 28 minSDK 21 targetsdk 28 and not work in my project in android pie, just in 8.1 and 7