Fix the existing translation doesn't update for new translated result.

It is possible the existing text is changed that triggers a new
translation. We cache the translation response when onShowTransltion
is called. We should keep the translation response is updated if this
is a new translation result.

Bug: 200232741
Test: atest CtsTranslationTestCases
Test: manual. The issue app works fine.
Change-Id: Iaf7423cd35d4484e33de84e478256b77a000f390