Fix showing translated text if user switch back to original lauguage quickly.

Fixes the bug if the translation service spending more time to deal
with the translation but user switches back to original lauguage
quickly, we will show translated text.

When the response back, if the translation state is finished, we drop
the result. If the state is pause, we update the response but we do
not call onShowTranslation to translated text.

Bug: 200251777
Test: atest CtsTranslationTestCases
Test: manual. Try to send request late and switch back original
language, it keeps original. Then it works fine when switch between
the source and target languages.

Change-Id: I780456f091884c61e5801ead3ff021531fee87b4