Fix several issues related to CircularIconsPreference

* Sometimes would cause an unending stream of accessibility events (particularly if starting off-screen). This would break TalkBack and anything that depends on UiAutomator (although the system itself took it like a champ).
* Sometimes would not load images (because the ViewTreeObserver would never fire onGlobalLayout after being added because measured width was 0, even though a new width was calculated later).
* Would not recalculate the number of icons that fit if the width changes after the first layout.

Combining ViewHolders with waiting for measuring and/or ViewTreeObservers was always a wonky approach, even though it should've worked in theory. This should be more robust.

Also fixes the unwanted animation on mode screen load related to the absence of the header name being applied a tad too late.

Fixes: 359948417
Fixes: 360072876
Fixes: 360328804
Test: atest SettingsRoboTests + manual + adb shell uiautomator events
Flag: android.app.modes_ui
Change-Id: I7e5dfbdab220d1ebc1c68e5e87ce544ee86b6a65