Eleven: bump minSdk to 28

Commit 78ddb450fa3fce19ae1389bbb489cfe9c3210de0 makes use of
AlbumColumns.ALBUM_ID instead of BaseColumns._ID, which breaks
on API levels below 28.

To prevent people from scratching their heads when trying to
run a recent version of Eleven on older API levels, bump the
minSdk to 28.

Change-Id: If89a8f34221058b4f4000bce4690327391cc820e
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>