Remove InputMethodManagerDelegate#isRestartOnNextWindowFocus

As isRestartOnNextWindowFocus is to check
InputMethodManager#mRestartOnNextWindowFocus flag if the input
connection is needed to force re-start when calling
startInputAsyncOnWindowFocusGain during gaining the window focus
from ImeFocusController#onPostWindowFocus next time.
(e.g. Switching IME apps from IME picker dialog)

Since in startInputOnWindowAyncFocusGain can just check
mRestartOnNextWindowFocus directly without exposing
isRestartOnNextWindowFocus, we could just simplied with inlining
the check logic.

Also, noticed in startInputAsyncOnWindowFocusGain has already reverted
CL[1]'s asynchronize starting input logic for some reasons, as a result,
renaming to startInputOnWindowFocusGain in case misleading.

[1]: I6aa4a664cfd0c86f75cee2457715317194bbe5e2

Bug: 244504062
Test: atest CtsInputMethodTestCases
Change-Id: I9335262136950021efb7b4b73c7eeb930e1333bb