Ensure Keyguard Preview Renderer is created on the main thread

When in guest mode, keyguardPreviewRenderer is created not on the main
thread, which is causing crashes due to various dependent classes
requiring construction on the main thread.

This is causing a silent crash in wallpaper picker preventing users from
seeing a preview of the lock screen.

The stack trace does not make it possible to track all of these down
individually and fix it at the source of the class.

This fix is behind a flag to be able to ensure that the performance is
satisfactory before progressing.

Bug: 343732179
Test: atest CustomizationProviderTest.kt
Flag: com.android.systemui.lockscreen_preview_renderer_create_on_main_thread
Change-Id: I6da9b473d53e2326fdf217a6f7a2540c0bb66dd8