downloader: update downloader api
downloader: update downloader api

rename some methods
add docstrings
add sparse image detection
fix some runtime errors

Signed-off-by: Young-Ho Cha <f1f8667db13e02d63e57947edf3fdfaa80cd9a11@gmail.com>

ganadist/adb_downloader,ganadist/adb_downloader,ganadist/adb_downloader