Enable resource shrinking for SystemUI

This is already in use with several SystemUI derivatives and provides
demonstrable size improvements. Enable use when SystemUI optimizations
are enabled, in anticipation of using the base SystemUI java_defaults
for downstream variants for consistency.

Bug: 202959019
Test: m + presubmit
Change-Id: I20d25d3d8cd73fa082b43b8666f332a58ba32cdb