Enable dexpreopt and dex stripping in soong jars

Dexpreopt jars from soong, which also strips classes.dex from
javalib.jar.

Test: m -j checkbuild
Test: unzip -l $OUT/system/framework/core-oj.jar
Change-Id: I06b8efa8993f6c44f0ad8c988e57a3cba08e17ad