Disable dexpreopt on targets that do not include default ART config.

Dexpreopt build commands should only be generated on targets that
include build/make/target/product/runtime_libart.mk, which sets the
necessary variables such as PRODUCT_SYSTEM_DEFAULT_PROPERTIES that
contain default values passed to dex2oat. This file also sets a
variable PRODUCT_USES_DEFAULT_ART_CONFIG that is used in
build/make/core/dex_preopt.mk to decide if boot images should be
installed.

On some targets build/make/target/product/runtime_libart.mk is not
included. Prior to this patch, on such targets invalid dexpreopt
commands were generated, but not used, so they did not cause any
visible build failures. The invalid commands can be grepped as:

  lunch qemu_trusty_arm64-userdebug && m nothing \
    && fgrep -e '-Xms ' $ANDROID_BUILD_TOP/out/soong/build.ninja

In this case '-Xms ' is an ill-formed option passed to dex2oat (the
option expects one argument, but none is passed).

This patch makes the DisablePreopt variable passed from make to
soong more strict: it not only requires WITH_DEXPREOPT, but also
PRODUCT_USES_DEFAULT_ART_CONFIG. This means that dexpreopt commands
will not be generated on targets that do not include
build/make/target/product/runtime_libart.mk.

Test: lunch aosp_walleye-userdebug && m \
    && find $ANDROID_BUILD_TOP/out -name 'boot.art'
    # expect to find files in /out/target/product/walleye/system/framework/$ARCH/

Test: lunch qemu_trusty_arm64-userdebug && m \
    && fgrep -e '-Xms ' $ANDROID_BUILD_TOP/out/soong/build.ninja
    # expect empty output

Change-Id: I3d765ed0dd8b38236b8bdd5c6202bb1d3f45f904