Translate: Text alpha animation on toggling translated state.

Apply a simple text alpha animation when toggling between original and
translated text. The text is fully faded out, then swapped to the new
text, then the fading is reversed.

Quick toggles are handled by ending the previous animation (which resets
the alpha value) when starting a new one. If the toggle is extremely
fast (<250ms for the currently defined animation duration), the text
stays in it's original state instead of swapping back-and-forth. This is
arguably not ideal, but anyway not worth the complexity for fixing.

There is an unhandled edge case where if the color is changed by the app
during the animation, the app's change would get overridden. It should
be rare and doesn't seem very important to fix.

Bug: 178651829
Test: atest CtsTranslationTestCases
Test: manual - across several apps, while scrolling, concurrently
 interacting with the app, concurrently closing the app, selecting text
 during the animation (with animation speed slowed down to see the
 effect).
Change-Id: I08a26de2253bb345f01186a6748b2d0ff6c2a419