Add PRODUCT_BOOT_JARS to derive BOOTCLASSPATH

- BOOTCLASSPATH now is product-configurable;
- No need to maintain the duplicate values in core/dex_preopt.mk.
- clean up some legacy ALL_PREBUILTs.
Bug: 9990214

Change-Id: Ie3953e66d282e335bb7782b0ebd56102c35ec10e