ota_from_target_files: Include the payload properties.

When building an A/B OTA package, include the payload.bin properties as
a key-value pairs text file, so it can easily be passed to
update_engine during payload application.

Bug: 26991255
TEST=`ota_from_target_files out/dist/${BOARD}-target_files.zip full-ota.zip` includes the properties.

Change-Id: I445c8a8e412a8e16b48b6ee626db8e27d48a38a9