[Central Surfaces] Make LetterboxBackgroundProvider a singleton.

See bug for more information.

Bug: 277762009
Test: compiles
Test: `adb shell dumpsys activity service
com.android.systemui/.SystemUIService LetterboxBackgroundProvider` ->
dumps letterbox provider info
Test: atest LetterboxBackgroundProviderTest

Change-Id: Id589c335d19468d3320b8a33ea0ed5d2246e0f5b