[Chromecast] Refactor ConnectivityChecker for test.

Make ConnectivityChecker virtual and add default implementation.
Add FakeConnectivityChecker for use in tests.

TEST=Build cast_shell. Build/run internal unittests.

BUG=

Review URL: https://codereview.chromium.org/1152293006

Cr-Commit-Position: refs/heads/master@{#332499}
8 files changed