Refine the ViewTranslationCallback usage.

Currently, TextView uses its default implementation even developers
uses setViewTranslationCallback() to set their customized
ViewTranslationCallback, we should only set default TextView
implementation if developers don't set it.

The onViewTranslationResponse() will call getViewTranslationCallback
instead of getting TextView default implementation directly. This can
make sure we can get the expected ViewTranslationCallback.

Bug: 183467275
Test: manual
Test: atest CtsTranslationTestCases

Change-Id: I41417140f8985aec6c80f1bca3cfba804727d5df