Fix product variables with no soong.variables

If soong.variables didn't exist, loadFromConfigFile would write default
values to soong.variables, but return with the product variables set to
the zero values.  Replace jsonConfigurable.DefaultConfig() with
SetDefaultConfig() that modifies the current object, and call it before
writing the values.

Change-Id: I7b7404c7a51975dc4493e25c775b3cf56ef335e3
2 files changed