Enable SystemUI R8 optimizations by default

This has been the default setting for Pixel/Go/Cuttlefish devices since
tm-dev. Broaden the default to enable for all devices against the base
SystemUI target. Note that packages overriding the base SystemUI target
may choose to ignore this flag and set their own optimization settings.

Opting out of optimizations can be achieved with either:
  * Env: export SYSTEMUI_OPTIMIZE_JAVA=false
  * Make: SYSTEMUI_OPTIMIZE_JAVA := false

Bug: 203472868
Test: m + presubmit + validate dex on new lunch targets
Change-Id: Id97031cc2c0d1d614391c404850002faf57bd4ac