Fix PullUidProvider unregistering on config update

Previously, MetricsManagers would unregister themselves as a
PullUidProvider for a given ConfigKey in the destructor. This caused all
pulls to fail after a config update because the new MetricsManager would
register itself before the old MetricsManager was destructed and
unregistered. This resulted in the old MetricsManager removing the new
config since they shared the same config key. The fix is for the
PullerManager to check that the PullUidProviders are equal in the
unregister function before actually erasing it.

Test: bit statsd_test:* (wrote a failing test that now passes).
Test: statsd_localdrive to manually update a config, ensured pulls still
worked.
Bug: 154544328

Change-Id: Id7af3b3b407e24bee74fc34bd1c2b9e0575e9c9e