Consider IME_VISIBLE_IMPERCEPTIBLE as shown for taskbar purposes

IME_VISIBLE isn't set until the keyboard is "perceptibly" visible to the user (e.g. based on alpha). But we want to signal to taskbar as soon as we know IME will be visible.

Test: tap an edit field to open keyboard, ensure taskbar reacts immediately instead of halfway through the transition
Bug: 202511986
Change-Id: I39c15578ed3e5b729f4645ad76988b113f0b57a6