Polish new IME API for L: CursorAnchorInfo

This CL allows application authors and input method authors to
communicate with each other more precisely on the visibility of
insertion marker and composing characters. Now we can describe
the situation where the coordinates of them are available but
they are overlapped by other UI elements.

This change is based on feedbacks from internal customers of
this preview API.

Change-Id: I82eba0e844a6f8b99ba11a68fad272399034cc24
BUG: 16118303