Add OnDevice to TranslationManager method names.

Makes the API easier to evolve in case we add user-selectable
TranslationServices later, like we do for RecognitionService.

Also makes the create.. name consistent with
SpeechRecognizer#createOnDeviceSpeechRecognizer().

Bug: 176208267
Test: atest CtsTranslationTestCases
Change-Id: I1a66bd045202025b9c5f669018c989654bcaa1ad