DynamicPartitionControl: only create snapshot when snapshot_enabled

Do not create snapshot when applying downgrade
to non-Virtual-A/B packages and secondary OTAs.

Test: apply downgrade OTA on Virtual A/B devices

Bug: 138733621 (secondary OTA)
Fixes: 138258570 (downgrades)

Change-Id: I13318f57613d6bd60a5b7e81ebb3e35b3c225a0c
diff --git a/boot_control_android_unittest.cc b/boot_control_android_unittest.cc
index f090de2..e44af15 100644
--- a/boot_control_android_unittest.cc
+++ b/boot_control_android_unittest.cc
@@ -197,6 +197,9 @@
                {T("system"), 2_GiB},
                {T("vendor"), 1_GiB}});
 
+  EXPECT_CALL(dynamicControl(), PreparePartitionsForUpdate(_, _, _, false))
+      .WillOnce(Return(true));
+
   EXPECT_TRUE(PreparePartitionsForUpdate(
       target(), {{"system", 2_GiB}, {"vendor", 1_GiB}}, false));