libsnapshot: Add a source_partition parameter to OpenSnapshotWriter.

Reading from the base device is incorrect, because if the partition
shrinks, we may still have copy operations from the removed area in the
original partition. Ask the caller to explicitly name the source device
for AddCopy() operations.

Bug: 168554689
Test: vts_libsnapshot_test
Test: full OTA with update_device.py
Test: incremental OTA with update_device.py
Change-Id: If388e37c2a2f9288a43d2849312c921bf59d4918