Use runtime config file instead of compile flag.

Use runtime config file instead of compile flag to control an
optional external grpc server address. This allows us to test
connecting with an external simluated power unit without recompiling
the image.

Test: Manual test with push the config file at
/vendor/etc/automotive/powercontroller/serverconfig
Bug: 325675883

Change-Id: I04193e7603a670c8398c6d6dc0466a0071f7c33d