Add horizontal padding to empty shade text.

Since the view is centered, this is only visible when the text is very
long (so essentially only when the modes_ui_empty_shade is on or the
text size is very large). It now looks better for longer text.

Bug: 366003631
Test: manually tested that it looks good including when the footer is
visible in the empty shade
Flag: EXEMPT minor xml change

Change-Id: Ieface63b6c7f6fcfb5dd63432b4c5598b3eb646a