Add artd unit tests.

This change adds `art_artd_tests`, which contains the unit tests of
artd. The change also splits the code into artd.h/artd.cc and
artd_main.cc, where artd.h/artd.cc contains the implementation that is
unit-testable, while artd_main.cc contains the main function of the artd
binary.

Note: The test is not enabled because it currently doesn't work on
master-art.

Bug: 177273468
Test: art/tools/buildbot-build.sh
Test: m art-check-testing-apex-gen
Change-Id: I6b73174422e000071f5d41bc31164aa3ee1d5301
Merged-In: I19701d7ca83f541becdcd413e740eff93d03037d
9 files changed