Use isDisplayed instead of isOnScreen

In DisplayPolicy#shouldBeHiddenByKeyguard: WindowState#isOnScreen uses more checks that are not needed when verifying if the IME should be hidden. Use WindowState#isDisplayed instead.

Test: com.android.server.wm.flicker.ime.ShowImeOnUnlockScreenTest#visibleLayersShownMoreThanOneConsecutiveEntry
Fix: 374413957
Flag: EXEMPT refactor
Change-Id: I851aa29f11aacb36b6d47bc74e599e14a756a331