Add UiTranslationStateCallback.

This is primarily intended for Input Methods, so they can offer
complementary translation experiences to the UI Translation.

But it could also be useful to the applications being translated too, so
it's added to UiTranslationManager.

The states are intentionally different from the ones in the View API -
these are about the UI Translation feature lifecycle, whereas the View
states are about the display states (shown/hidden).

Bug: 177500482
Test: manual - with a sample IME
CTS-Coverage-Bug: 182853869
Change-Id: I8255e18c29a229c8ea3b89098b76124ee8955bdc