[automerger skipped] Support optional prop assignments am: d721e870 am:...

[automerger skipped] Support optional prop assignments am: d721e870 am: 54403635 -s ours am: ec1d03db -s ours

am skip reason: Change-Id I9c073a21c8257987cf2378012cadaeeeb698a4fb with SHA-1 598ea498 is in history

Original change: https://android-review.googlesource.com/c/platform/build/+/1344820

Change-Id: I7aca05a3840c7fc39e88d90d749e4de07846fea5