[metrics] Always enable background thread

Having the thread enabled in some cases and not in others was causing
issues this tests. To avoid needing to handle multiple configurations,
this CL enables the background reporting thread always. In general,
metrics reporting will be enabled, so this better reflects the default
state of things.

Test: ./test.py --run-test --host
Bug: 170149255
Change-Id: I5100e0da1a7d264ebdd1bfe60f08171ffa5b73b1
5 files changed