Implement TranslationCallback with support for partial responses.

Translator#translate() is also updated to be asynchronous with callback
instead.

Bug: 178651352
Test: atest CtsTranslationTestCases
CTS-Coverage-Bug: 178651352
Change-Id: Ibe1f97994fe6ee593a3a3bbb452c075b49a7eb5a
8 files changed