Configure existing ThemePicker Robolectric tests to run with atest.

Submitting here first to resolve conflicts from upstream branches, will
cherry-pick into aosp-master afterwards.

For now, broken tests are marked @Ignore.

Also moves Robolectric tests into directory under tests/ for similar
structure to other test code in the wider codebase.

Bug: 260777356
Bug: 260925899
Test: this is the test
Change-Id: I25c4efdb73dc5eb88611efac669de13fa2be0c3d