acloud: To change download remote image directory.
1. Let user change default download directory if available space is
insufficient.
2. add argument --image-download-dir
Bug: 117614779
Test: atest acloud_test, m acloud && acloud create --local-instance &&
acloud create --local-instance --image-download-dir ~/testdl &&
acloud create --local-instance --image-download-dir ~/testdl --branch aosp-master --build-target aosp_cf_x86_phone-userdebug --build-id 5102275 -vv
Change-Id: If8dffaa491c2f56b319c148be45e0b9a3e328020
4 files changed