Refactor LetterboxScrollProcessor

This refactors LetterboxScrollProcessor so that all feature related
logic are inside the class and now that InputEventCompatProcessor just
calls its method.

This is a preparation of InputEventCompatProcessor refactoring.

Bug: 369865835
Test: LetterboxScrollProcessorTest InputEventCompatProcessorTest
Flag: EXEMPT refactor
Change-Id: I7424c2778413d8c9d0e47bb30a720d64a2d9e829