Thumbnail capture AL fix

On AL devices, windows not currently visible cannot have a thumbnail
captured. This change causes thumbnail fetching to fall back to the
latest cached thumbnail in such cases.

Test: atest SystemUITests:com.android.systemui.screencapture
BUG: 39894067
Flag: com.android.systemui.large_screen_sharing
Change-Id: I833d171d4a286d070c08c376d57053c54d37ba45