metricsd: Make the unit tests pass.

This cleans up the unit tests and update them to pass.

Cleaned up:
* removed the irrelevant bits.
* Used ScopedTempDir for all test specific files (instead of the current
  directory).
* Update some objects to make them more easily testable.
* Group all the test in a single binary and use bionic's test runner.

BUG: 23682444
Change-Id: I289e3a5ff89968fdecd4a156e93bc38bbc25f58b