Create separate target for CastSysInfoAndroidThings for testing.

Bug: internal b/120633112
Test: Ran unit tests
Change-Id: Idbda6d8ff9b26db40b924dc1bf9d56ab331d4e8e
Reviewed-on: https://chromium-review.googlesource.com/c/1366389
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Alex Leung <alexleung@google.com>
Cr-Commit-Position: refs/heads/master@{#615608}