Bump up target version to API 28
Bump up target version to API 28

The related manifest change was already checked in thanks to @ericafenyo
(#616)

shankari/e-mission-phone,e-mission/e-mission-phone,e-mission/e-mission-phone,shankari/e-mission-phone,e-mission/e-mission-phone,e-mission/e-mission-phone,shankari/e-mission-phone,shankari/e-mission-phone