Add API for TranslationContext

* TranslationContext holds source/dest specs, replacing spec pairs in
methods.
* TranslationCapability holds information on the translation models, as well
as a helper method to generate a TranslationContext for creating
translators.
* TCapability is meant to hold information about the translation models, and
provide information/flag on what the translator can support, versus the
TContext which will indicate exactly which supported flags should be
used and activated by the models.
* Added TM.getTranslationCapabilities, and add/remove TCapabilityUpdateListener

Bug: 176208267
Test: atest CtsTranslationTestCases
Change-Id: I2ab7a3eadcbbc9e13f8f33bf9c51cda69f30fdb7