Remove TODO in InputMethodStressTest

Since CL[1] has fixed WIC#hide(ime()) not work in onCreate,
now we can remove the comment of verifyHideBehavior back for following
test methods in ImeOpenCloseStressTest to ensure the test coverage:

- testShowHideWithInputMethodManager_onCreate
- testHideWithWindowInsetsController_onCraate_requestFocus
- testShowHideWithInputMethodManager_inSameFrame

Also, in case calling show/hide IME consecutively when in
Activity#onCreate that comes before the window focus changed and the
final IME visiblity may be affected by SOFT_INPUT_STATE_.* visiblity
flag after gaining the window focus, ensure setting
SOFT_INPUT_STATE_UNCHANGED flag in following tests:

- testHideWithWindowInsetsController_onCreate_requestFocus
- testShowHideWithInputMethodManager_onCreate

[1]: Id3aebca3348888826bc5d3c6694e94cddb80b4d4

Fix: 248456059
Test: atest InputMethodStressTest:ImeOpenCloseStressTest

Change-Id: Ib7baf28036f01201176d2a5bba4ebde319ca8f1a