Fix pass null TranslationCapability to dispatchRequestTranslation()

We only provide text-text capability now and when the UiTranslation
APIs are called, the model files should be ready. To simplify the call
flow, we hardcode the TranslationCapability now. If more capabilities
are supported, we will call the API to get the TranslationCapability.

Bug: 183589662
Test: atest CtsTranslationTestCases

Change-Id: Iea1baac7988402298b2577d03adf9d889f45ad53