Remove miniresolver header elevation

Header elevation caused a line to be drawn beneath the header.

Fix: 323482191
Test: manual testing: launch miniresolver and check that there's no line
 beneath the header in both the light and the dark theme.
Change-Id: Ia047e4b3c39a5991387c64374d2379bcab708b75
Merged-In: Ia047e4b3c39a5991387c64374d2379bcab708b75
(cherry picked from commit ac92b12e)