Rename collectors-helper to statsd-helper.

The collectors-helper library should be more aptly named statsd-helper.
This CL renames the helper library and adds statsd-helper-test. We will
remove collectors-helper-test once this CL has merged internally and all
uses of it are replaced with statsd-helper-test.

Bug: 119891007
Test: build internal master w/ CL
Change-Id: I44a3ecbaf21af7a4b52a01129c90e7bc4ea8f7c7