aconfig: include namespace in create-device-config-defaults

Update the output format of create-device-config-defaults to include the
flag's namespace. Also change the delimiters. The new format is

  <namespace>:<package>.<flag-name>=[enabled|disabled|

Bug: 285468565
Test: atest aconfig.test
Change-Id: I9b4ca1611cca8528dc341fc12812b614c86f6c08