Add unit tests for TargetService

This change introduces `TargetServiceTest.java` to verify the behavior
of `DataImporterService`. The implementation was also changed to make
certain methods and variables visible for testing.

Integration tests for the data importer service will be added in a
subsequent CL due to their complexity.

Bug: 436171659
Change-Id: Icadd3f0d2ce70fef0fae0dd231776c0995f813dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6841255
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Michael Tatarski <mtatarski@google.com>
Auto-Submit: Michael Tatarski <mtatarski@google.com>
Cr-Commit-Position: refs/heads/main@{#1502581}