aconfig: add create-device-config-defaults command

DeviceConfig is the backend for READ_WRITE flags.

Add a new command "create-device-config-defaults" to create a file that
DeviceConfig will read to pre-populate its data store on first init.

This will be used to quickly switch flag values during CI tests:
rebuilding and reflashing a device would have the same effect, but would
be costlier. This feature is not expected to be used outside CI tests.

Note: because DeviceConfig only works with READ_WRITE flags, the
generated file excludes READ_ONLY flags.

Bug: 285468565
Test: atest aconfig.test
Change-Id: I4caff1a10647b8da0ce4e3615678993a957a92dd