BootControlAndroid::GetPartitionDevice checks super metadata

... before checking device mapper. When sideloading, current (source) slot
partitions aren't mapped on device mapper in recovery. Hence, When
GetPartitionDevice is called:

- If device is already mapped, use it
- If device isn't mapped but is in metadata, this is a dynamic
  partition and will be mapped (with force_writable = false, since
  writable partitions should have been mapped with force_writable
  in InitPartitionMetadata)
- If device isn't mapped and it is not in metadata, this is a static
  partition.

Test: manual sideload incremental update with a device with dynamic partitions
Test: update_engine_unittests

Fixes: 117101719
Change-Id: I85b490e11cee689846d2bf899b6015f1b320bed9