Support cuttlefish with an emulated 2GB SD

Currently we have an assumption that SD card's size is bigger
than the internal /data so the DSU would  always choose SD as the
destination when possible. This assumption is not valid for
cuttlefish which only have an emulated 2G SD

Bug: 179980369
Bug: 171861574

Test: A physical device with a SD card inserted and
      CF local instance with an emulated SD

Change-Id: I15c5d38ab316b8d65546d876fe235d0c9c6ed6aa