Set IME showing in ImeInsetsSourceProvider#onPostLayout

The snapshot logic relies on the information about the IME being shown or not. In the initial refactor, this was not correctly set which led to the IME snapshot not being shown.

Test: atest FlickerTestsIme3:com.android.server.wm.flicker.ime.ShowImeOnUnlockScreenTest#imeAndAppAnimateTogetherWhenLockingAndUnlocking
Flag: android.view.inputmethod.refactor_insets_controller
Fix: 355052042
Change-Id: I118749534c824f1f375f3aa43d2fa5b13e0c3222