add course list activity UI for android-L
fix bug where the course list ui not shown properly
fix bug so that user will not go back to login screen when press back button in course list ui
course list from cloud is shown correcly in the ui
user can logout and go back to login screen now
fix compatibility issues by removing android-L support
fix course list padding so that the list look more compact
fix system bar tint problem
Merge branch 'feature/course_list' into develop