Add Soong variable for optimizing SystemUI Java

Introduce a Soong variable to allow selective enabling of Java
optimizations for SystemUI. This is analogous to the
`SYSTEM_OPTIMIZE_JAVA` flag, but scoped just to SystemUI,
decoupling the feature from system server-related development.

Opting in to optimizations can be achieved with either:
  * Env:
export SYSTEMUI_OPTIMIZE_JAVA=true
  * Make:
$(call add_soong_config_var_value,ANDROID,SYSTEMUI_OPTIMIZE_JAVA,true)

Note that the build rule changes to hook into this variable will
land separately.

Bug: 203088572
Test: `SYSTEMUI_OPTIMIZE_JAVA=true m` (observe env variable change log)
Change-Id: I24b4b76cdf844e3c00735e19e69898867162c81d
diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index cd42e1f..78ac809 100644
--- a/core/android_soong_config_vars.mk
+++ b/core/android_soong_config_vars.mk
@@ -100,6 +100,9 @@
 $(call add_soong_config_var_value,ANDROID,module_build_from_source,true)
 endif
 
+# TODO(b/203088572): Remove when Java optimizations enabled by default for
+# SystemUI.
+$(call add_soong_config_var,ANDROID,SYSTEMUI_OPTIMIZE_JAVA)
 # TODO(b/196084106): Remove when Java optimizations enabled by default for
 # system packages.
 $(call add_soong_config_var,ANDROID,SYSTEM_OPTIMIZE_JAVA)