Treat R8 warnings as failures for SystemUI

Treating warnings as failures avoids unexpected issues at build and
runtime, particularly for missing class references. Eventually this
will be the default setting, but for now explicitly opt in critical
system targets with no warnings.

Test: m SystemUI
Bug: 242088131
Change-Id: I7eeddb2244a04c7bb7de7c1f16d6d38ad0137c59