aconfig: add create-device-config-sysprops command

Add a new "create-device-config-sysprops" command that works like
"create-device-config-defaults" but for system properties.

DeviceConfig is a Java service, and will mirror (some of) its values by
setting system properties in the persist.device_config namespace. Native
code will access DeviceConfig (actually, the system properties) via the
server_configurable_flags library.

The new command writes a file that can be appended to /system/build.prop
to pre-populate persist.device_config before DeviceConfig has started.

Like create-device-config-defaults, the new command skips READ_ONLY
flags.

Bug: 285468565
Test: atest aconfig.test
Change-Id: I311c7c5e0b03dc897b09204137d43cc182324717