Fetch a/b ota partitions from the build prop, instead of walking through
/dev/block/by-name

Test: administer a gki partial update on cuttlefish
Bug: 162148770
Change-Id: I173bd3ee1c462428ed02a9421c87ebed8dde636d
diff --git a/common/fake_boot_control.h b/common/fake_boot_control.h
index adbacd6..5d8823a 100644
--- a/common/fake_boot_control.h
+++ b/common/fake_boot_control.h
@@ -57,6 +57,9 @@
     if (part_it == devices_[slot].end())
       return false;
     *device = part_it->second;
+    if (is_dynamic != nullptr) {
+      *is_dynamic = false;
+    }
     return true;
   }