Backup: fix 'out of memory' issue when backing up large partition as image

Restore: improve image restoration speed

* Workaround for devices that need to backup partition as image, eg: dm-verity
* Increased block size used for partition restore, now using 8M instead of 4K

Change-Id: Ie7e7ff3283d71a7e459d76f72caea9f57e71c628