Refactor: Extract splash alpha logic from TaskThumbnailViewModel

This commit updates how splash alpha is propagated to `TaskThumbnailView`. These changes reduces the usage of flows to handle animations to improve the performance by avoiding inherent overhead for sending messages between threads (go/sysui-flow-overhead).

Bug: 390581380
Doc: go/launcher-overview-unified-taskviewmodel
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: OverviewImageTest
Test: TaskViewModelTest
Test: TaskThumbnailViewModelImplTest
Test: TaskThumbnailViewScreenshotTest
Change-Id: I467da1a90b653fb108eb32534bc0dd2a67d95083