Initialize DEX_PREOPT_DEFAULT based on global dexpreopt settings.

Bug: 177214554
Test: lunch cf_x86_phone-userdebug && m
Test: Ensure that `find out -name dexpreopt.config` lists before and
    after the fix are identical (for target cf_x86_phone-userdebug).
Test: TARGET_PRODUCT=mainline_modules_x86_64 \
    TARGET_BUILD_VARIANT=userdebug \
    build/soong/soong_ui.bash --make-mode UipbUtil
Change-Id: I28b38c08ad570d53dc679f9b42975545d457416e