Handle IME when target moves between splits

Handle split-screen IME when target window is launched in secondary
from primary split.
By comparing actual InputMethod's input target to the window requesting
IME window, we can allow showInsets(ime()).

Fix: 159576146
Test: atest ImeInsetsSourceProviderTest
Change-Id: Ife4137cc84eba99cf4f6e2f3957f13dcb95818c9