Allow same matcher to cancel >1 event activation

Previously,we could only have one deactivation matcher for a given
metric. This meant that the same matcher could not be used to cancel
several (or all) event activations of a metric. This fixes that bug.

Test: bit statsd_test:*
Bug: 134185513
Change-Id: I9d74878c9e09bd1f58c8f3dcffe1092edf490329