Do not dump metrics if reproxy was never started

Dumping of RBE metrics should only happen when reproxy was started.
Reproxy is started only when Soong is run in make / build mode and not
in dumpvars mode.

Doc: https://docs.google.com/document/d/1Iqhu5QgozB_rA9fucMHO6X4U6A8e8qFtZyub9j83ICQ/edit#
Test: Ran a build with / without RBE. No metrics dumped during lunch
without RBE and metrics dumping worked as expected with RBE.
Bug: b/197213143

Change-Id: Ic3403331e1f4ac2c7c72fd9169aa7b82a7545c75