Allow flag-guarded Java optimizations for SystemUI

Introduce a set of optional Java optimization settings for
SystemUI, based on the Soong variable added in aosp/1896612.

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

As these optimizations can change the resulting stack traces, a parallel
effort is working to simplify retracing of stack traces for more
accurate debugging and diagnostics. Additional stabilization and testing
will occur before any effort to enable these optimizations by default
for specific targets. Also note that there are no plans to enable
obfuscation.

Preliminary results (w/ `SYSTEM_OPTIMIZE_JAVA=true`):
  * SystemUI.apk (31MB -> 26MB)

Bug: 203472868
Test: SYSTEM_OPTIMIZE_JAVA=true m (validate SystemUI.apk)
Change-Id: Idd8d5ef32c06e992ad35846eb5fab67ecb58830a