MetadataBuilder::NewForUpdate: check sysprop instead

Check the value of ro.boot.dynamic_partitions_retrofit instead of
checking the name of the super partition being 'super' is a more
reliable way of determining retrofit DAP devices.

Some devices launch with DAP (e.g. cuttlefish) doesn't have "super"
as the super partition name. When Virtual A/B is implemented on
cuttlefish, update_engine calls NewForUpdate for the current super
partition metadata. Hence, this code needs to check the retrofit
sysprop instead.

Also, renamed IsRetrofitDevice to IsRetrofitMetadata to avoid the
confusion.

Test: OTA on retrofit DAP device
Test: OTA on launch DAP device
Test: liblp_test_static
Change-Id: I4636de854734df1bb61779d9a955217e89fdb2fd