Fix two bugs with thumbnail screenshotting for split apps

This patch fixes two bugs with the thumbnail screenshotting flow for split apps. There is no currently filed bug behavior for these bugs, but they look wrong in the code -- this is the proposed fix/refactor.

Fixes: 285191462
Test: Still passes presubmit
Change-Id: I43eefa334d66676b8175919e266b82947f1837e5