Add ability to toggle themed icons (1/3)

Refactor the old themed icons workflow to use clean architecture and
Kotlin flows. Replace usage of ThemedIconSectionController with MVVM
architecture. Also, instead of using ThemedIconSwitchProvider and
ThemedIconUtils to get themed icon state from the content provider,
consolidate all the functionality into ThemedIconRepository, and create
a flow of the themed icon state using content observer.

There is still a noticeable delay for the Launcher preview to update.
Also, additional UI updates are needed. These will be addressed in
follow-up CLs.

Flag: com.android.systemui.shared.new_customization_picker_ui
Test: manually verified
Bug: 383867754
Bug: 376091518
Change-Id: I4753ad64fdbb5e8e9145760ea37f2e0675043975