[Notif redesign] Change top margin calculation

Although obtainStyledAttributes is a valid way of obtaining the font
size, it's a final method and causing a bunch of issues with tests that
use a mocked/fake context and try to build notifications. Let's use the
size dimension directly instead.

Bug: 396421144
Test: CarrierServiceStateTrackerTest
Flag: android.app.notifications_redesign_templates
Change-Id: I3e4367eb5aeee2cc5fc4bcaa74f63a0e87c356c1