Make MockIME optional.

MockIME is not available in some devices, so we need to fall back to "native" IME,
otherwise the tests won't run.

Test: atest CtsAutoFillServiceTestCases:android.autofillservice.cts.DatasetFilteringTest#testFilter_usingKeyboard
Bug: 137963114

Change-Id: Iea8a903b6fba7917da2a88d8f4ec67120fc825d5