Update SystemUI build flag for optimization

Use the new `SYSTEMUI_OPTIMIZE_JAVA` build flag instead, decoupling
optimization opt-in from system_server. Note that this remains
opt-in and disabled by default.

Bug: 203472868
Test: SYSTEMUI_OPTIMIZE_JAVA=true m SystemUI (validate SystemUI.apk)
Change-Id: Ib0482324561939da806707aea860d040e9aa81ef