Deduplicate config generation for boot images.

Both default and apex configs are used for dexpreopting bootclasspath
jars. The difference between the two configs is in the naming (default
paths contain "boot", and apex paths contain "apex"). Another
difference is that apex config does not have a zip archive.

Test: m
Test: `find $ANDROID_BUILD_TOP/out -name '*.art' | sort`
    returns the same list of files before and after the patch

Change-Id: I5b2e8d83ab94fd0b1b3d4dc3f0db243ef70bfb08