Improved storage size detection.

The total storage size was calculating by reading and multiplying the
contents of /sys/block/mmcblk0/size and
/sys/block/mmcblk0/queue/hw_sector_size.

On some devices, such calculation doesn't work because:

1.The primary block is not /sys/block/mmcblk0 .
2.The sector size is not the right value to use.

These 2 issues are temporarily addressed by providing alternative
primary blocks and hardcoding the size (512 bytes). In the long term,
the size should be calculated by vold, so each device could provide its
own calculation if necessary.

BUG: 30216622
Change-Id: I8f9a9f4f753d3c92bab9257062b61ed2b9d665c5
Fixes: 24128505