libsnapshot: Properly set partial_update_

On GKI updates, has_dynamic_partition_metadata() may be false. Even if
it is the case, partial_update_ should be set properly.

Test: apply GKI update
Bug: 162616968

Change-Id: Icf055d8eb3060e36b3e977541a24f62f9fe11a6f
1 file changed