Call InitPartitionMetadata when resuming an update.

BootControlAndroid::InitPartitionMetadata() should always be called when
starting / resuming an update that uses dynamic partitions. This allows
updating the metadata for the target slot, as well as setting up the
internal state for accessing the dynamic partitions in the target slot.

Bug: 122097567
Test: Run update_engine_unittests on taimen and blueline.
Test: Apply a payload that uses dynamic partitions. Reboot before it
      finishes, then resume the update.
Change-Id: I1353f7460a61c0183654b6349689beaa9bf12129