Add test for providing IME insets to freeform windows

Adds a unit test for `InsetsPolicy#enforceInsetsPolicyForTarget` to
verify that IME insets are correctly provided to floating windows.

The test covers the following scenarios:
- A window in the same task as the IME target receives IME insets.
- The IME target window itself receives IME insets.
- A window in a different task does not receive IME insets.
- No window receives IME insets if the IME is not requested to be
  visible.
- Pinned (Picture-in-Picture) windows do not receive IME insets.

Please help fill out the survey for feedback: https://docs.google.com/forms/d/e/1FAIpQLSeKFKpHImCAqZIa_OR801cw72HQUreM2oGM25C3mKKT2tBFnw/viewform?usp=pp_url&entry.1586624956=ag/35495073

Please feel free to make changes to the generated tests based on your domain knowledge. More context about the project please see go/ai-testgen

Original Change: ag/35419508 (Note tests are based on the original CL but may add coverage beyond the specific changes to improve overall code health)

Test: ATP tests passed http://go/forrest-run/L34900030017616815
Bug: 431235865
Flag: TEST_ONLY

Change-Id: Ia93cf86c77444a366e52292c4d54b0e9646aba37