Fix hub empty state after hub layout changes

With the hub layout expanded to 3 rows on mobile, the empty state CTA
tile was aligned at the top of the screen and also did not perform well
with display/font size turned up. This CL does some minimal adjustments
to make the UI acceptable for a11y purposes.

Bug: 431148916
Fixes: 431148916
Test: manual, see bug for before/after comparison
Flag: NONE small UI fix
Change-Id: I8d36cb41065dffed559d194d21b498223e69aea6