Add test for InputMethodSubtypeSwitchingController#getItems

Adds a unit test for the `getItems` method to verify its filtering logic based on the `forMenu` and `includeAuxiliary` parameters.

This improves test coverage for a public method that was previously untested.


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Original Change: ag/34663935 (Note that tests are based on the original CL but may add coverage beyond the specific changes to improve overall code health, especially when the original change restructured the code)
Test: ATP tests passed http://go/forrest-run/L89200030017347546
Bug: 431235865
Flag: TEST_ONLY

Change-Id: Iaf1c6d87163c65219f74b75dfa2629ee384d10ee