DeepShortcut - show loading placeholder before content is available.

Sometimes the deep shortcuts can take time to load, show an initial loading
state before any text is set.

Note: changed singleLine to lines=1 as the former causes issues with RTL
layout and is deprecated.

Bug: 78620208
Tested: manual test on pixel 3

Change-Id: I782addb8d64425a1a95cee4f104fc0bd44809815