Recycle thumbnailViews in DesktopTaskView

- Changed mTaskContainers to a List
- Added an XML for TaskThumbnailView to make it compatible with ViewPool
- Removed mSnapshotViews variable, and streamlined DesktopTaskView binding logic to re-use thumbnailvIew from mTaskContainers or ViewPool

Bug: 249371338
Fix: 251586230
Test: manual
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_mode DEVELOPMENT
Change-Id: Iab7575e9c36b8ebf3eb62f19f13d75ba671f9f4f