metricsd: Fix race condition in the tests.

We cached the enabled status for metrics as a static field in the
metrics library. This field is not reset in between tests which treats
the metrics as enabled if they were enabled previously.

None of the users rely on the caching being static so the cached status
was changed into a field.
Also enabled the metrics in upload_service_test.cc after verifying that
the test failed consistently.

BUG: 24473349
TEST: unit tests.

Change-Id: If05fdca644016d923aa7b1978db5e74ada563c1a