Add product_variables.eng.optimize.enabled

The default build is to enable optimization for all of module.
In order to have the feasibility to the owners of the modules,
to create product_variables.eng.optimize.enabled let the owners
to have the choice to decide whether the build enable the optimization
or not.

Test: atest -m # build soong
Test: # follow Developing for Soong & setup GOPATH
      # to run the new tests in variables_test.go
    cd $GOPATH; \
    go test android/soong/android
Test: #1. change frameworks/base/packages/SystemUI/Android.bp to change
          product_variables.eng.optimize.enabled = false;
      #2. source build/setupenv.sh;
      #3. lunch aosp_x86-eng; make -j16 SystemUI
      #4. lunch aosp_x86-userdebug; make -j16 SystemUI
      #5. aosp_x86-eng should be bigger than aosp_x86-userdebug
Bug: 130259652

Change-Id: Ie62c35458ca91d66ec65bda58d315fb971139849