Switch to SystemUI dimen for notification margins.

In an upcoming review, I'll be updating notification_content_margin_start
to 56dp (while leaving _end at 16dp).  This is for an intended asymmetry
in the new notification design, but was causing unintended asymmetry in
Quick Settings and notification guts.

Test: atest SystemUITests
Test: manual inspection
Change-Id: Ic2c4f4ffe0aa9f2e4caee8e6e85bf65435b9d969