Improve caching behavior of thumbnails

Remove the retrieval cache on system_server. It's not needed at
this point.

Instead, we cache the low-res thumbnails on SystemUI side that
will be visible when recents launches. For that, we introduce
a strong thumbnail cache, which gets filled up whenever the task
stack changes.

Also fix a couple of issues like that the visibility report was
pretty wrong as well as some tasks got unloaded because tasks were
bound before layout happend.

Also fix a merge issue where we didn't load the reduced resolution
thumbnail :/

Test: TaskSnapshotCacheTest
Test: Open a couple of apps, open recents, make sure all
thumbnails are already loaded.

Fixes: 36374895
Change-Id: Idbf1acd4ceab6a7c4656e9791e245a8b102017f2