Migrate statsd_test to use libstatslog_statsdtest

This is part of the migration to remove libstatslog from statsd

Since we still depend on libstatsmetadata, we cannot fully mock out
all the atoms. So we will create a new libstatslog_statsdtest to house
atoms used only in the tests.

Bug: 150976524
Test: bit statsd_test:*
Change-Id: I6368305eb89b2c35e670e42907a308afd922e604