Fix the Translator references are gone when receiving response.

There are some issues TranslationService already sent the response
but the View onResponse doesn't be called. The reference seems to be
gone. If we don't fix the problem, we will missing the ui translation
even the service already give us the response. To fix the problem, we
change it to strong reference.

The change is low risk, the only risk is if TranslationService doesn
not release the reference to the callback properly causing a memory
leak for apps but this can recovered by app update.

Bug: 194973014
Test: atest CtsTranslationTestCases
Test: The translation for chat apps work fine. The memory is decreased
after receiving the responses for a while.

Change-Id: Ib3956b2250c54a29acf9f7d51f51e8191fe8aba2