Hack aosp_riscv64 config to fix `m droid dist`

`m droid dist` fails on hiddenapi rules missing bootclasspath classes,
add default_art_config.mk to set PRODUCT_BOOT_JARS, and then override
PRODUCT_USES_DEFAULT_ART_CONFIG back to false to avoid a dependency
from the boot.zip dist artifact to dex2oat, which doesn't work for
riscv64 yet.

Test: lunch aosp_riscv64-userdebug && m droid dist
Change-Id: Icc332ee9132568251386ec742932f6708312bb92